fixup flutter gpu.

impeller/renderer/backend/metal/render_pass_mtl.mm

@@ -105,7 +105,7 @@ static bool ConfigureStencilAttachment(
   auto result = [MTLRenderPassDescriptor renderPassDescriptor];
 
   const ColorAttachment& color0 = desc.GetColor0();
-  if (!ConfigureColorAttachment(color0, result.colorAttachments[0u])) {
+  if (!ConfigureColorAttachment(color0, result.colorAttachments[0])) {
     VALIDATION_LOG << "Could not configure color attachment at index 0";
     return nil;
   }

impeller/renderer/render_target.cc

@@ -221,6 +221,10 @@ ColorAttachment RenderTarget::GetColor0() const {
   return ColorAttachment{};
 }
 
+bool RenderTarget::HasColor0() const {
+  return color0_.has_value();
+}
+
 const std::map<size_t, ColorAttachment>& RenderTarget::GetColorAttachments()
     const {
   return colors_;

impeller/renderer/render_target.h

@@ -111,6 +111,8 @@ class RenderTarget final {
 
   ColorAttachment GetColor0() const;
 
+  bool HasColor0() const;
+
   const std::map<size_t, ColorAttachment>& GetColorAttachments() const;
 
   const std::optional<DepthAttachment>& GetDepthAttachment() const;

lib/gpu/render_pass.cc

@@ -105,6 +105,10 @@ RenderPass::GetOrCreatePipeline() {
 
   pipeline_desc.SetSampleCount(render_target_.GetSampleCount());
 
+  if (render_target_.HasColor0()) {
+    auto& color = GetColorAttachmentDescriptor(0);
+    color.format = render_target_.GetRenderTargetPixelFormat();
+  }
   for (const auto& it : render_target_.GetColorAttachments()) {
     auto& color = GetColorAttachmentDescriptor(it.first);
     color.format = render_target_.GetRenderTargetPixelFormat();