[Impeller] Fix and reland drawPaint collapsing optimization. (#43097)

Collapses DrawPaint calls into clear colors when possible.

issue flutter/flutter#129292
relands #41711

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Author: gaaclarke

impeller/aiks/aiks_unittests.cc

@@ -1992,6 +1992,18 @@ TEST_P(AiksTest, OpacityPeepHoleApplicationTest) {
   ASSERT_TRUE(delegate->CanCollapseIntoParentPass(entity_pass.get()));
 }
 
+TEST_P(AiksTest, DrawPaintAbsorbsClears) {
+  Canvas canvas;
+  canvas.DrawPaint({.color = Color::Red(), .blend_mode = BlendMode::kSource});
+  canvas.DrawPaint(
+      {.color = Color::CornflowerBlue(), .blend_mode = BlendMode::kSource});
+
+  Picture picture = canvas.EndRecordingAsPicture();
+
+  ASSERT_EQ(picture.pass->GetElementCount(), 0u);
+  ASSERT_EQ(picture.pass->GetClearColor(), Color::CornflowerBlue());
+}
+
 TEST_P(AiksTest, ForegroundBlendSubpassCollapseOptimization) {
   Canvas canvas;
 

impeller/aiks/canvas.cc

@@ -172,6 +172,16 @@ void Canvas::DrawPath(const Path& path, const Paint& paint) {
 }
 
 void Canvas::DrawPaint(const Paint& paint) {
+  if (xformation_stack_.size() == 1 &&  // If we're recording the root pass,
+      GetCurrentPass().GetElementCount() == 0 &&  // and this is the first item,
+      (paint.blend_mode == BlendMode::kSourceOver ||
+       paint.blend_mode == BlendMode::kSource) &&
+      paint.color.alpha >= 1.0f) {
+    // Then we can absorb this drawPaint as the clear color of the pass.
+    GetCurrentPass().SetClearColor(paint.color);
+    return;
+  }
+
   Entity entity;
   entity.SetTransformation(GetCurrentTransformation());
   entity.SetStencilDepth(GetStencilDepth());

impeller/entity/entity_pass.cc

@@ -478,7 +478,7 @@ EntityPass::EntityResult EntityPass::GetEntityForElement(
         renderer,                                  // renderer
         subpass_size,                              // size
         subpass->GetTotalPassReads(renderer) > 0,  // readable
-        clear_color_.Premultiply());               // clear_color
+        Color::BlackTransparent());                // clear_color
 
     if (!subpass_target.IsValid()) {
       VALIDATION_LOG << "Subpass render target is invalid.";

impeller/renderer/render_target.cc

@@ -313,7 +313,7 @@ RenderTarget RenderTarget::CreateOffscreenMSAA(
   // Color attachment.
 
   ColorAttachment color0;
-  color0.clear_color = Color::BlackTransparent();
+  color0.clear_color = color_attachment_config.clear_color;
   color0.load_action = color_attachment_config.load_action;
   color0.store_action = color_attachment_config.store_action;
   color0.texture = color0_msaa_tex;