Fix: Metal: EEVEE Next viewport motion blur

Resolves assertion for EEVEE Next motion
blur wherein a swizzled texture used in an
image binding loses write-access. We
instead must bind the source texture
for image write operations.

This is now consistent with expected
behaviour in other APIs.

Authored by Apple: Michael Parkin-White

Pull Request: https://projects.blender.org/blender/blender/pulls/117479

source/blender/gpu/metal/mtl_context.mm

@@ -1883,6 +1883,12 @@ void MTLContext::ensure_texture_bindings(
              * shader. */
             id<MTLTexture> tex = bound_texture->get_metal_handle();
 
+            /* If texture resource is an image binding and has a non-default swizzle mask, we need
+             * to bind the source texture resource to retain image write access. */
+            if (!is_resource_sampler && bound_texture->has_custom_swizzle()) {
+              tex = bound_texture->get_metal_handle_base();
+            }
+
             if (bool(shader_texture_info.stage_mask & ShaderStage::COMPUTE)) {
               cs.bind_compute_texture(tex, slot);
               cs.bind_compute_sampler(bound_sampler, use_argument_buffer_for_samplers, slot);

source/blender/gpu/metal/mtl_texture.hh

@@ -291,6 +291,14 @@ class MTLTexture : public Texture {
     return name_;
   }
 
+  bool has_custom_swizzle()
+  {
+    return (mtl_swizzle_mask_.red != MTLTextureSwizzleRed ||
+            mtl_swizzle_mask_.green != MTLTextureSwizzleGreen ||
+            mtl_swizzle_mask_.blue != MTLTextureSwizzleBlue ||
+            mtl_swizzle_mask_.alpha != MTLTextureSwizzleAlpha);
+  }
+
   id<MTLBuffer> get_vertex_buffer() const
   {
     if (resource_mode_ == MTL_TEXTURE_MODE_VBO) {