[Impeller] perform final blit and gpu end frame tracing earlier. (flutter/engine#54452)

Fixes the reported GPU time regression when rendering platform views. The problem was that we delayed the tracing of the end of the frame until the CATransaction, which artificially stretches out GPU frame time. Instead add a new method SurfaceMTL::PreparePResent which performs the final blit and tracing, call this in the encode callback.

Fixes: https://flutter-flutter-perf.skia.org/e/?queries=sub_result%3Daverage_gpu_frame_time%26test%3Dplatform_views_scroll_perf_ios__timeline_summary&selected=commit%3D41853%26name%3D%252Carch%253Dintel%252Cbranch%253Dmaster%252Cconfig%253Ddefault%252Cdevice_type%253DiPhone_11%252Cdevice_version%253Dnone%252Chost_type%253Dmac%252Csub_result%253Daverage_gpu_frame_time%252Ctest%253Dplatform_views_scroll_perf_ios__timeline_summary%252C

Test: benchmarks.

engine/src/flutter/impeller/renderer/backend/metal/surface_mtl.h

@@ -65,6 +65,9 @@ class SurfaceMTL final : public Surface {
     present_with_transaction_ = present_with_transaction;
   }
 
+  /// @brief Perform the final blit and trigger end of frame workloads.
+  bool PreparePresent();
+
   // |Surface|
   bool Present() const override;
 
@@ -82,6 +85,7 @@ class SurfaceMTL final : public Surface {
   std::optional<IRect> clip_rect_;
   bool frame_boundary_ = false;
   bool present_with_transaction_ = false;
+  bool prepared_ = false;
 
   static bool ShouldPerformPartialRepaint(std::optional<IRect> damage_rect);
 

engine/src/flutter/impeller/renderer/backend/metal/surface_mtl.mm

@@ -222,8 +222,7 @@ IRect SurfaceMTL::coverage() const {
   return IRect::MakeSize(resolve_texture_->GetSize());
 }
 
-// |Surface|
-bool SurfaceMTL::Present() const {
+bool SurfaceMTL::PreparePresent() {
   auto context = context_.lock();
   if (!context) {
     return false;
@@ -260,6 +259,17 @@ bool SurfaceMTL::Present() const {
 #ifdef IMPELLER_DEBUG
   ContextMTL::Cast(context.get())->GetGPUTracer()->MarkFrameEnd();
 #endif  // IMPELLER_DEBUG
+  prepared_ = true;
+  return true;
+}
+
+// |Surface|
+bool SurfaceMTL::Present() const {
+  FML_CHECK(prepared_);
+  auto context = context_.lock();
+  if (!context) {
+    return false;
+  }
 
   if (drawable_) {
     id<MTLCommandBuffer> command_buffer =

engine/src/flutter/shell/gpu/gpu_surface_metal_impeller.mm

@@ -157,6 +157,9 @@ std::unique_ptr<SurfaceFrame> GPUSurfaceMetalImpeller::AcquireFrameFromCAMetalLa
         surface->PresentWithTransaction(surface_frame.submit_info().present_with_transaction);
 
         if (clip_rect && clip_rect->IsEmpty()) {
+          if (!surface->PreparePresent()) {
+            return false;
+          }
           surface_frame.set_user_data(std::move(surface));
           return true;
         }
@@ -167,7 +170,6 @@ std::unique_ptr<SurfaceFrame> GPUSurfaceMetalImpeller::AcquireFrameFromCAMetalLa
         const impeller::RenderTarget& render_target = surface->GetTargetRenderPassDescriptor();
         surface->SetFrameBoundary(surface_frame.submit_info().frame_boundary);
 
-        surface_frame.set_user_data(std::move(surface));
 #if EXPERIMENTAL_CANVAS
         impeller::TextFrameDispatcher collector(aiks_context->GetContentContext(),
                                                 impeller::Matrix());
@@ -181,13 +183,25 @@ std::unique_ptr<SurfaceFrame> GPUSurfaceMetalImpeller::AcquireFrameFromCAMetalLa
         impeller_dispatcher.FinishRecording();
         aiks_context->GetContentContext().GetTransientsBuffer().Reset();
         aiks_context->GetContentContext().GetLazyGlyphAtlas()->ResetTextFrames();
+
+        if (!surface->PreparePresent()) {
+          return false;
+        }
+        surface_frame.set_user_data(std::move(surface));
         return true;
 #else
         impeller::DlDispatcher impeller_dispatcher(cull_rect);
         display_list->Dispatch(impeller_dispatcher, sk_cull_rect);
         auto picture = impeller_dispatcher.EndRecordingAsPicture();
         const bool reset_host_buffer = surface_frame.submit_info().frame_boundary;
-        return aiks_context->Render(picture, render_target, reset_host_buffer);
+
+        auto result = aiks_context->Render(picture, render_target, reset_host_buffer);
+
+        if (!surface->PreparePresent()) {
+          return false;
+        }
+        surface_frame.set_user_data(std::move(surface));
+        return result;
 #endif
       });
 
@@ -278,6 +292,9 @@ std::unique_ptr<SurfaceFrame> GPUSurfaceMetalImpeller::AcquireFrameFromMTLTextur
         surface->PresentWithTransaction(surface_frame.submit_info().present_with_transaction);
 
         if (clip_rect && clip_rect->IsEmpty()) {
+          if (!surface->PreparePresent()) {
+            return false;
+          }
           return surface->Present();
         }
 
@@ -296,6 +313,9 @@ std::unique_ptr<SurfaceFrame> GPUSurfaceMetalImpeller::AcquireFrameFromMTLTextur
         impeller_dispatcher.FinishRecording();
         aiks_context->GetContentContext().GetTransientsBuffer().Reset();
         aiks_context->GetContentContext().GetLazyGlyphAtlas()->ResetTextFrames();
+        if (!surface->PreparePresent()) {
+          return false;
+        }
         bool render_result = true;
 #else
         impeller::DlDispatcher impeller_dispatcher(cull_rect);
@@ -310,8 +330,7 @@ std::unique_ptr<SurfaceFrame> GPUSurfaceMetalImpeller::AcquireFrameFromMTLTextur
           FML_LOG(ERROR) << "Failed to render Impeller frame";
           return false;
         }
-
-        return true;
+        return surface->PreparePresent();
       });
 
   SurfaceFrame::SubmitCallback submit_callback =