[Impeller] Ensure that missing color attachment 0u does not cause crash in embedder API (flutter#43705)

flutter/flutter#130619

Author: Jonah Williams

impeller/entity/entity_pass.cc

@@ -247,7 +247,8 @@ bool EntityPass::Render(ContentContext& renderer,
                         const RenderTarget& render_target) const {
   auto root_render_target = render_target;
 
-  if (root_render_target.GetColorAttachments().empty()) {
+  if (root_render_target.GetColorAttachments().find(0u) ==
+      root_render_target.GetColorAttachments().end()) {
     VALIDATION_LOG << "The root RenderTarget must have a color attachment.";
     return false;
   }
@@ -341,7 +342,7 @@ bool EntityPass::Render(ContentContext& renderer,
 
   // The safety check for fetching this color attachment is at the beginning of
   // this method.
-  auto color0 = root_render_target.GetColorAttachments().find(0)->second;
+  auto color0 = root_render_target.GetColorAttachments().find(0u)->second;
 
   // If a root stencil was provided by the caller, then verify that it has a
   // configuration which can be used to render this pass.

shell/gpu/gpu_surface_gl_impeller.cc

@@ -83,7 +83,6 @@ std::unique_ptr<SurfaceFrame> GPUSurfaceGLImpeller::AcquireFrame(
   GLFrameInfo frame_info = {static_cast<uint32_t>(size.width()),
                             static_cast<uint32_t>(size.height())};
   const GLFBOInfo fbo_info = delegate_->GLContextFBO(frame_info);
-
   auto surface = impeller::SurfaceGLES::WrapFBO(
       impeller_context_,                            // context
       swap_callback,                                // swap_callback

shell/platform/embedder/embedder.cc

@@ -999,7 +999,7 @@ MakeRenderTargetFromBackingStoreImpeller(
 
   impeller::RenderTarget render_target_desc;
 
-  render_target_desc.SetColorAttachment(color0, framebuffer->target);
+  render_target_desc.SetColorAttachment(color0, 0u);
   render_target_desc.SetStencilAttachment(stencil0);
 
   return std::make_unique<flutter::EmbedderRenderTargetImpeller>(