Fix lighting volume WGSL depth texture binding

LightingVolume binds the shadow generator's shadow map
depthStencilTexture - a depth-format texture - as the shadowMap input of
the lightingVolume compute shader. Declare that binding as
texture_depth_2d instead of texture_2d<f32>, and drop the .r swizzle on
the textureLoad result since textureLoad on texture_depth_2d returns a
scalar f32. The loaded value is the same depth that .r previously
carried, so far-plane fitting results are unchanged.

This completes the compute depth sample-type support introduced in
PR BabylonJS#18460: _GetComputeTextureSampleType classifies this texture as bind
group layout sampleType "depth", and WebGPU validation requires a
texture_depth_2d WGSL declaration for a "depth" layout entry. With the
previous texture_2d<f32> declaration, createComputePipeline fails
validation (observed in Chromium and wgpu-native) as soon as the
lighting volume compute shaders run with an explicit pipeline layout.
The change is also safe with the default auto layout: texture_depth_2d
derives sampleType "depth", which is valid for the depth-aspect view
Babylon binds, so behavior is identical there.

This came from the Hill Valley GLTF/NativeXR AR Portal validation pass.

(cherry picked from commit a81ccfb)

Author: matthargett

packages/dev/core/src/ShadersWGSL/lightingVolume.compute.fx

@@ -8,7 +8,7 @@ struct Params {
     orthoMax: vec3f,
 };
 
-@group(0) @binding(0) var shadowMap : texture_2d<f32>;
+@group(0) @binding(0) var shadowMap : texture_depth_2d;
 @group(0) @binding(1) var<uniform> params : Params;
 @group(0) @binding(2) var<storage,read_write> positions : array<f32>;
 
@@ -27,7 +27,7 @@ fn updateFarPlaneVertices(@builtin(global_invocation_id) global_id : vec3u) {
     let stepY = floor(params.step * f32(coord.y));
     let depthCoord = vec2u(u32(floor(f32(coord.x) * params.step)), u32(stepY));
 
-    var depth = textureLoad(shadowMap, depthCoord, 0).r;
+    var depth = textureLoad(shadowMap, depthCoord, 0);
 #ifdef MOVE_FAR_DEPTH_TO_NEAR
     if (depth == 1.0) {
         depth = 0.0;