[Impeller] cache render target properties on Render Pass. (flutter/engine#48323)

Otherwise we do three hashmap lookups everything we call OptionsFromPass(AndEntity) in a contents.

## Before
16 / 392 = 4%

![image](https://github.com/flutter/engine/assets/8975114/cf02778c-22da-4849-a98a-75bac7536284)

## After
6/ 458 = 1.3%

![image](https://github.com/flutter/engine/assets/8975114/13b58267-a25a-483d-a632-55f505070996)

engine/src/flutter/impeller/entity/contents/contents.cc

@@ -19,11 +19,9 @@ namespace impeller {
 
 ContentContextOptions OptionsFromPass(const RenderPass& pass) {
   ContentContextOptions opts;
-  opts.sample_count = pass.GetRenderTarget().GetSampleCount();
-  opts.color_attachment_pixel_format =
-      pass.GetRenderTarget().GetRenderTargetPixelFormat();
-  opts.has_stencil_attachment =
-      pass.GetRenderTarget().GetStencilAttachment().has_value();
+  opts.sample_count = pass.GetSampleCount();
+  opts.color_attachment_pixel_format = pass.GetRenderTargetPixelFormat();
+  opts.has_stencil_attachment = pass.HasStencilAttachment();
   return opts;
 }
 

engine/src/flutter/impeller/renderer/render_pass.cc

@@ -9,6 +9,10 @@ namespace impeller {
 RenderPass::RenderPass(std::weak_ptr<const Context> context,
                        const RenderTarget& target)
     : context_(std::move(context)),
+      sample_count_(target.GetSampleCount()),
+      pixel_format_(target.GetRenderTargetPixelFormat()),
+      has_stencil_attachment_(target.GetStencilAttachment().has_value()),
+      render_target_size_(target.GetRenderTargetSize()),
       render_target_(target),
       transients_buffer_() {
   auto strong_context = context_.lock();
@@ -23,12 +27,24 @@ RenderPass::~RenderPass() {
   }
 }
 
+SampleCount RenderPass::GetSampleCount() const {
+  return sample_count_;
+}
+
+PixelFormat RenderPass::GetRenderTargetPixelFormat() const {
+  return pixel_format_;
+}
+
+bool RenderPass::HasStencilAttachment() const {
+  return has_stencil_attachment_;
+}
+
 const RenderTarget& RenderPass::GetRenderTarget() const {
   return render_target_;
 }
 
 ISize RenderPass::GetRenderTargetSize() const {
-  return render_target_.GetRenderTargetSize();
+  return render_target_size_;
 }
 
 HostBuffer& RenderPass::GetTransientsBuffer() {

engine/src/flutter/impeller/renderer/render_pass.h

@@ -6,6 +6,7 @@
 
 #include <string>
 
+#include "impeller/core/formats.h"
 #include "impeller/renderer/command.h"
 #include "impeller/renderer/command_buffer.h"
 #include "impeller/renderer/render_target.h"
@@ -71,8 +72,29 @@ class RenderPass {
   ///
   const std::vector<Command>& GetCommands() const { return commands_; }
 
+  //----------------------------------------------------------------------------
+  /// @brief      The sample count of the attached render target.
+  SampleCount GetSampleCount() const;
+
+  //----------------------------------------------------------------------------
+  /// @brief      The pixel format of the attached render target.
+  PixelFormat GetRenderTargetPixelFormat() const;
+
+  //----------------------------------------------------------------------------
+  /// @brief      Whether the render target has an stencil attachment.
+  bool HasStencilAttachment() const;
+
  protected:
   const std::weak_ptr<const Context> context_;
+  // The following properties: sample_count, pixel_format,
+  // has_stencil_attachment, and render_target_size are cached on the
+  // RenderTarget to speed up numerous lookups during rendering. This is safe as
+  // the RenderTarget itself is copied into the RenderTarget and only exposed as
+  // a const reference.
+  const SampleCount sample_count_;
+  const PixelFormat pixel_format_;
+  const bool has_stencil_attachment_;
+  const ISize render_target_size_;
   const RenderTarget render_target_;
   std::shared_ptr<HostBuffer> transients_buffer_;
   std::vector<Command> commands_;

engine/src/flutter/impeller/renderer/renderer_unittests.cc

@@ -1273,6 +1273,25 @@ TEST_P(RendererTest, CanPreAllocateCommands) {
   EXPECT_EQ(render_pass->GetCommands().capacity(), 100u);
 }
 
+TEST_P(RendererTest, CanLookupRenderTargetProperties) {
+  auto context = GetContext();
+  auto cmd_buffer = context->CreateCommandBuffer();
+  auto render_target_cache = std::make_shared<RenderTargetAllocator>(
+      GetContext()->GetResourceAllocator());
+
+  auto render_target =
+      RenderTarget::CreateOffscreen(*context, *render_target_cache, {100, 100});
+  auto render_pass = cmd_buffer->CreateRenderPass(render_target);
+
+  EXPECT_EQ(render_pass->GetSampleCount(), render_target.GetSampleCount());
+  EXPECT_EQ(render_pass->GetRenderTargetPixelFormat(),
+            render_target.GetRenderTargetPixelFormat());
+  EXPECT_EQ(render_pass->HasStencilAttachment(),
+            render_target.GetStencilAttachment().has_value());
+  EXPECT_EQ(render_pass->GetRenderTargetSize(),
+            render_target.GetRenderTargetSize());
+}
+
 }  // namespace testing
 }  // namespace impeller