MetalDevice: Support multisampling

src/util/metal_shaders.metal

@@ -0,0 +1,42 @@
+/// A custom resolve kernel that averages color at all sample points.
+#include <metal_stdlib>
+using namespace metal;
+
+// https://developer.apple.com/documentation/metal/metal_sample_code_library/improving_edge-rendering_quality_with_multisample_antialiasing_msaa?language=objc
+kernel void
+colorResolveKernel(texture2d_ms<float, access::read> multisampledTexture [[texture(0)]],
+                   texture2d<float, access::write> resolvedTexture [[texture(1)]],
+                   uint2 gid [[thread_position_in_grid]])
+{
+    const uint count = multisampledTexture.get_num_samples();
+    
+    float4 resolved_color = 0;
+    
+    for (uint i = 0; i < count; ++i)
+    {
+        resolved_color += multisampledTexture.read(gid, i);
+    }
+    
+    resolved_color /= count;
+    
+    resolvedTexture.write(resolved_color, gid);
+}
+
+kernel void
+depthResolveKernel(texture2d_ms<float, access::read> multisampledTexture [[texture(0)]],
+                   texture2d<float, access::write> resolvedTexture [[texture(1)]],
+                   uint2 gid [[thread_position_in_grid]])
+{
+  const uint count = multisampledTexture.get_num_samples();
+  
+  float resolved_depth = 0;
+  
+  for (uint i = 0; i < count; ++i)
+  {
+      resolved_depth += multisampledTexture.read(gid, i).r;
+  }
+  
+  resolved_depth /= count;
+  
+  resolvedTexture.write(float4(resolved_depth, 0, 0, 0), gid);
+}