metal: match pipeline attachment formats to the view

The Metal backend unconditionally configured every render pipeline
state for two color attachments (`for i in 0..2`) plus depth and
stencil at `Depth32Float_Stencil8`, while MTKView by default exposes
only color attachment 0 and `depthStencilPixelFormat = Invalid`. Metal
validation fires on the first draw:

    failed assertion `Set Render Pipeline State Validation
    For color attachment 1, the renderPipelineState pixelFormat must
    be MTLPixelFormatInvalid, as no texture is set.
    For depth attachment, the renderPipelineState pixelFormat must be
    MTLPixelFormatInvalid, as no texture is set.
    For stencil attachment, the renderPipelineState pixelFormat must
    be MTLPixelFormatInvalid, as no texture is set.

Configure only color attachment 0 (the backend has no MRT path) and
mirror the view's `depthStencilPixelFormat` onto the pipeline's depth
+ stencil formats — but only when the view actually has one. When
MTKView reports `Invalid`, skip the `set*AttachmentPixelFormat:` calls
so the descriptor keeps its `Invalid` defaults instead of having that
sentinel passed in explicitly.

Adds `MTLPixelFormat::Invalid = 0` to the binding (Apple's `MTLPixelFormatInvalid`).

Author: benface

src/graphics/metal.rs

@@ -988,8 +988,11 @@ impl RenderingBackend for MetalContext {
             msg_send_![descriptor, setVertexFunction:shader_internal.vertex_function];
             msg_send_![descriptor, setFragmentFunction:shader_internal.fragment_function];
             msg_send_![descriptor, setVertexDescriptor: vertex_descriptor];
+            // Only attachment 0 — the backend has no MRT path and
+            // setting attachment 1 to a non-Invalid pixelFormat
+            // fails Metal validation when no texture is bound.
             let color_attachments = msg_send_![descriptor, colorAttachments];
-            for i in 0..2 {
+            for i in 0..1usize {
                 let color_attachment = msg_send_![color_attachments, objectAtIndexedSubscript: i];
                 let view_pixel_format: MTLPixelFormat = msg_send![self.view, colorPixelFormat];
                 msg_send_![color_attachment, setPixelFormat: view_pixel_format];
@@ -1036,14 +1039,23 @@ impl RenderingBackend for MetalContext {
                     ];
                 }
             }
-            msg_send_![
-                descriptor,
-                setDepthAttachmentPixelFormat: MTLPixelFormat::Depth32Float_Stencil8
-            ];
-            msg_send_![
-                descriptor,
-                setStencilAttachmentPixelFormat: MTLPixelFormat::Depth32Float_Stencil8
-            ];
+            // Pipeline depth/stencil formats must match whatever
+            // render pass uses this pipeline. MTKView reports
+            // `Invalid` when no depth/stencil is configured —
+            // leave the descriptor's defaults (also `Invalid`)
+            // alone in that case; otherwise mirror the view.
+            let view_depth_stencil_format: MTLPixelFormat =
+                msg_send![self.view, depthStencilPixelFormat];
+            if view_depth_stencil_format != MTLPixelFormat::Invalid {
+                msg_send_![
+                    descriptor,
+                    setDepthAttachmentPixelFormat: view_depth_stencil_format
+                ];
+                msg_send_![
+                    descriptor,
+                    setStencilAttachmentPixelFormat: view_depth_stencil_format
+                ];
+            }
 
             let mut error: ObjcId = nil;
             let pipeline_state: ObjcId = msg_send![

src/native/apple/frameworks.rs

@@ -632,6 +632,7 @@ impl MTLClearColor {
 #[allow(non_camel_case_types)]
 #[derive(Debug, Copy, Clone, Hash, PartialEq, Eq)]
 pub enum MTLPixelFormat {
+    Invalid = 0,
     BGRA8Unorm = 80,
     Depth32Float = 252,
     Stencil8 = 253,