feat(shaders): add signed distance field vector texture shaders and documentation

- Introduced `sdf_vector_frag.glsl` for advanced SDF rendering with features like alpha testing, soft edges, outlining, and outer glow.
- Added `sdf_vector_simple_frag.glsl` for a simplified shader version with basic alpha testing and outlining.
- Created `sdf_vector_vert.glsl` for vertex processing in SDF rendering.
- Documented the SDF vector texture rendering technique in `SDF_VECTOR_TEXTURES.md`, detailing usage, parameters, and examples.

Author: timfox

src/shaders/SDF_VECTOR_TEXTURES.md

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+# Signed Distance Field Vector Texture Rendering
+
+## Overview
+
+This implementation provides high-quality rendering of vector art (text, UI elements, decals) using signed distance fields stored in low-resolution textures. The technique allows crisp rendering at any magnification level while using minimal texture memory.
+
+## Shaders
+
+### `sdf_vector_frag.glsl` / `sdf_vector_vert.glsl`
+Full-featured implementation with:
+- Alpha testing with configurable threshold (default 0.5)
+- Soft edge antialiasing with adaptive screen-space derivatives
+- Outlining with configurable width and color
+- Outer glow / drop shadows
+- Multi-channel SDF support for sharp corners
+- Optional base color texture
+
+### `sdf_vector_simple_frag.glsl`
+Simplified version closely matching the paper's HLSL code:
+- Basic alpha testing
+- Soft edges
+- Outlining
+- Outer glow / drop shadows
+
+## Usage
+
+### Basic Setup
+
+1. **Generate SDF Texture**: Use the compute shader `sdf_generate.comp` to convert a high-resolution binary image (e.g., 4096x4096) into a low-resolution SDF texture (e.g., 64x64).
+
+2. **Store Distance Field**: The signed distance field should be stored in the alpha channel of an 8-bit texture:
+   - `0.0` = maximum negative distance (inside)
+   - `0.5` = edge position
+   - `1.0` = maximum positive distance (outside)
+
+3. **Render**: Use the SDF shader with the generated texture.
+
+### Uniform Parameters
+
+#### Alpha Testing
+```glsl
+float alphaThreshold = 0.5;  // Edge position
+bool useAlphaTest = true;    // Use binary alpha test
+```
+
+#### Soft Edges / Antialiasing
+```glsl
+bool useSoftEdges = true;
+float softEdgeMin = 0.45;    // Start of soft region
+float softEdgeMax = 0.55;    // End of soft region
+bool useAdaptiveAA = true;   // Use screen-space derivatives
+```
+
+#### Outlining
+```glsl
+bool useOutline = true;
+float outlineMinValue0 = 0.48;  // Inner threshold
+float outlineMinValue1 = 0.49;  // Inner fade start
+float outlineMaxValue0 = 0.51;  // Outer fade end
+float outlineMaxValue1 = 0.52;  // Outer threshold
+vec4 outlineColor = vec4(0.0, 0.0, 0.0, 1.0);  // Black outline
+```
+
+#### Outer Glow / Drop Shadow
+```glsl
+bool useOuterGlow = true;
+vec2 glowUVOffset = vec2(0.01, 0.01);  // Shadow offset
+float outerGlowMinDValue = 0.0;        // Glow start
+float outerGlowMaxDValue = 0.4;         // Glow end
+vec4 outerGlowColor = vec4(0.0, 0.0, 0.0, 0.5);  // Semi-transparent shadow
+```
+
+#### Multi-Channel SDF (Sharp Corners)
+```glsl
+bool useMultiChannel = true;
+int channelCount = 2;  // Use red and alpha channels
+```
+
+When using multiple channels, the shader performs a logical AND operation:
+- Channel 1 (alpha): Primary distance field
+- Channel 2 (red): Secondary edge for sharp corners
+- Channel 3 (green): Tertiary edge
+- Channel 4 (blue): Quaternary edge
+
+## Examples
+
+### Simple Alpha Test (No Shader Required)
+For the simplest case, you can use hardware alpha testing with threshold 0.5:
+```glsl
+// In fragment shader
+float dist = texture(sdfTexture, texCoord).a;
+if (dist < 0.5) discard;
+```
+
+### Soft Edges
+```glsl
+float dist = texture(sdfTexture, texCoord).a;
+float alpha = smoothstep(0.45, 0.55, dist);
+fragColor.a *= alpha;
+```
+
+### Outlined Text
+```glsl
+float dist = texture(sdfTexture, texCoord).a;
+vec4 color = baseColor;
+
+// Draw outline in distance range [0.48, 0.52]
+if (dist >= 0.48 && dist <= 0.52) {
+    float factor = smoothstep(0.48, 0.49, dist) * 
+                   smoothstep(0.52, 0.51, dist);
+    color = mix(color, outlineColor, factor);
+}
+
+// Draw main shape
+color.a *= smoothstep(0.45, 0.55, dist);
+```
+
+### Drop Shadow
+```glsl
+float dist = texture(sdfTexture, texCoord).a;
+float shadowDist = texture(sdfTexture, texCoord + shadowOffset).a;
+
+// Draw shadow
+vec4 shadow = shadowColor * smoothstep(0.0, 0.4, shadowDist);
+
+// Draw main shape
+vec4 main = baseColor * smoothstep(0.45, 0.55, dist);
+
+// Composite
+fragColor = mix(shadow, main, step(0.5, dist));
+```
+
+## Performance Notes
+
+- **Memory**: A 64x64 SDF texture can represent vector art that would require 4096x4096 in traditional formats (64x memory savings)
+- **Performance**: Minimal overhead - typically 2-5 additional shader instructions
+- **Compatibility**: Works on all modern GPUs, including those without programmable shaders (using alpha testing)
+
+## References
+
+- Green, C. (2007). "Improved Alpha-Tested Magnification for Vector Textures and Special Effects". GPU Gems 3, Chapter 25.
+- Valve Corporation. Source Engine.

src/shaders/sdf_vector_frag.glsl

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+// Signed Distance Field Vector Texture Rendering
+//
+// This shader implements:
+// - Alpha testing with distance field threshold
+// - Soft edge antialiasing
+// - Outlining
+// - Outer glow / drop shadows
+// - Multi-channel support for sharp corners
+
+#version 450
+
+layout(location = 0) in vec2 v_texCoord;
+layout(location = 0) out vec4 fragColor;
+
+// SDF texture sampler
+layout(binding = 0) uniform sampler2D sdfTexture;
+
+// Base color texture (optional - can use uniform color instead)
+layout(binding = 1) uniform sampler2D baseColorTexture;
+
+// Uniforms
+layout(binding = 2) uniform SDFParams {
+    // Alpha testing
+    float alphaThreshold;        // Default: 0.5 (edge position)
+    bool useAlphaTest;            // Use simple alpha test instead of soft edges
+    
+    // Soft edges / antialiasing
+    bool useSoftEdges;
+    float softEdgeMin;            // Distance threshold for soft edge start
+    float softEdgeMax;            // Distance threshold for soft edge end
+    
+    // Outlining
+    bool useOutline;
+    float outlineMinValue0;       // Inner outline threshold
+    float outlineMinValue1;       // Inner outline fade start
+    float outlineMaxValue0;       // Outer outline fade end
+    float outlineMaxValue1;       // Outer outline threshold
+    vec4 outlineColor;            // Outline color
+    
+    // Outer glow / drop shadow
+    bool useOuterGlow;
+    vec2 glowUVOffset;            // Offset for glow/shadow lookup
+    float outerGlowMinDValue;     // Minimum distance for glow
+    float outerGlowMaxDValue;     // Maximum distance for glow
+    vec4 outerGlowColor;          // Glow/shadow color
+    
+    // Base color (used if baseColorTexture is not bound)
+    vec4 baseColor;
+    
+    // Multi-channel SDF for sharp corners
+    bool useMultiChannel;         // Use multi-channel SDF (AND operation)
+    int channelCount;             // Number of channels to use (1-4)
+    
+    // Screen-space derivatives for adaptive antialiasing
+    bool useAdaptiveAA;           // Use screen-space derivatives for adaptive AA
+} sdfParams;
+
+// Default values
+const float DEFAULT_ALPHA_THRESHOLD = 0.5;
+const float DEFAULT_SOFT_EDGE_MIN = 0.45;
+const float DEFAULT_SOFT_EDGE_MAX = 0.55;
+const float DEFAULT_OUTLINE_MIN_0 = 0.48;
+const float DEFAULT_OUTLINE_MIN_1 = 0.49;
+const float DEFAULT_OUTLINE_MAX_0 = 0.51;
+const float DEFAULT_OUTLINE_MAX_1 = 0.52;
+
+void main() {
+    // Sample the SDF texture
+    vec4 sdfSample = texture(sdfTexture, v_texCoord);
+    
+    // Extract distance field value(s)
+    float distAlphaMask = sdfSample.a;  // Primary distance field in alpha channel
+    
+    // Multi-channel SDF for sharp corners (AND operation)
+    if (sdfParams.useMultiChannel) {
+        if (sdfParams.channelCount >= 2) {
+            distAlphaMask = min(distAlphaMask, sdfSample.r);
+        }
+        if (sdfParams.channelCount >= 3) {
+            distAlphaMask = min(distAlphaMask, sdfSample.g);
+        }
+        if (sdfParams.channelCount >= 4) {
+            distAlphaMask = min(distAlphaMask, sdfSample.b);
+        }
+    }
+    
+    // Adaptive antialiasing using screen-space derivatives
+    float softEdgeMin = sdfParams.softEdgeMin;
+    float softEdgeMax = sdfParams.softEdgeMax;
+    
+    if (sdfParams.useAdaptiveAA) {
+        // Calculate texture coordinate derivatives
+        vec2 texSize = vec2(textureSize(sdfTexture, 0));
+        float dx = max(abs(dFdx(v_texCoord.x * texSize.x)), 0.001);
+        float dy = max(abs(dFdy(v_texCoord.y * texSize.y)), 0.001);
+        float dxy = max(dx, dy);
+        
+        // Widen soft region when texture is minified to reduce aliasing
+        float scale = max(1.0 / dxy, 1.0);
+        float softWidth = (softEdgeMax - softEdgeMin) * scale;
+        float center = (softEdgeMin + softEdgeMax) * 0.5;
+        softEdgeMin = center - softWidth * 0.5;
+        softEdgeMax = center + softWidth * 0.5;
+    }
+    
+    // Sample base color
+    vec4 baseColor = sdfParams.baseColor;
+    if (textureSize(baseColorTexture, 0).x > 0) {
+        baseColor = texture(baseColorTexture, v_texCoord) * sdfParams.baseColor;
+    }
+    
+    // Apply outlining
+    if (sdfParams.useOutline) {
+        if (distAlphaMask >= sdfParams.outlineMinValue0 && 
+            distAlphaMask <= sdfParams.outlineMaxValue0) {
+            
+            float oFactor = 1.0;
+            
+            // Fade in from inner edge
+            if (distAlphaMask <= sdfParams.outlineMinValue1) {
+                oFactor = smoothstep(
+                    sdfParams.outlineMinValue0,
+                    sdfParams.outlineMinValue1,
+                    distAlphaMask
+                );
+            }
+            // Fade out to outer edge
+            else {
+                oFactor = smoothstep(
+                    sdfParams.outlineMaxValue1,
+                    sdfParams.outlineMaxValue0,
+                    distAlphaMask
+                );
+            }
+            
+            baseColor = mix(baseColor, sdfParams.outlineColor, oFactor);
+        }
+    }
+    
+    // Apply soft edges / antialiasing
+    if (sdfParams.useSoftEdges) {
+        baseColor.a *= smoothstep(softEdgeMin, softEdgeMax, distAlphaMask);
+    } else if (sdfParams.useAlphaTest) {
+        // Simple alpha test (binary on/off)
+        baseColor.a = (distAlphaMask >= sdfParams.alphaThreshold) ? 1.0 : 0.0;
+    } else {
+        // Default: use distance field directly as alpha
+        baseColor.a = distAlphaMask;
+    }
+    
+    // Apply outer glow / drop shadow
+    if (sdfParams.useOuterGlow) {
+        vec4 glowTexel = texture(sdfTexture, v_texCoord + sdfParams.glowUVOffset);
+        float glowDist = glowTexel.a;
+        
+        // Multi-channel support for glow
+        if (sdfParams.useMultiChannel) {
+            if (sdfParams.channelCount >= 2) {
+                glowDist = min(glowDist, glowTexel.r);
+            }
+            if (sdfParams.channelCount >= 3) {
+                glowDist = min(glowDist, glowTexel.g);
+            }
+            if (sdfParams.channelCount >= 4) {
+                glowDist = min(glowDist, glowTexel.b);
+            }
+        }
+        
+        float glowFactor = smoothstep(
+            sdfParams.outerGlowMinDValue,
+            sdfParams.outerGlowMaxDValue,
+            glowDist
+        );
+        
+        vec4 glowColor = sdfParams.outerGlowColor * glowFactor;
+        
+        // Blend glow behind the main shape
+        float maskUsed = (baseColor.a > 0.0) ? 1.0 : 0.0;
+        baseColor = mix(glowColor, baseColor, maskUsed);
+    }
+    
+    fragColor = baseColor;
+}