[Impeller] fixes behavior for blurred rounded rect clear (#46167)

fixes: flutter/flutter#132849

I decided to mimic skia's blend function for clear only in the case for
blurred rounded rects since that fixes the listed issue. I'm not sure if
there are are other cases where we'd want this behavior. I've added
golden tests for the other clear cases too to make sure we didn't break
them.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide] and the [C++,
Objective-C, Java style guides].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I added new tests to check the change I am making or feature I am
adding, or the PR is [test-exempt]. See [testing the engine] for
instructions on writing and running engine tests.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I signed the [CLA].
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#overview
[Tree Hygiene]: https://github.com/flutter/flutter/wiki/Tree-hygiene
[test-exempt]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[C++, Objective-C, Java style guides]:
https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
[testing the engine]:
https://github.com/flutter/flutter/wiki/Testing-the-engine
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#handling-breaking-changes
[Discord]: https://github.com/flutter/flutter/wiki/Chat

Author: gaaclarke

impeller/aiks/aiks_unittests.cc

@@ -3581,5 +3581,37 @@ TEST_P(AiksTest, VerticesGeometryUVPositionData) {
   ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
 }
 
+TEST_P(AiksTest, ClearBlendWithBlur) {
+  Canvas canvas;
+  Paint white;
+  white.color = Color::Blue();
+  canvas.DrawRect(Rect::MakeXYWH(0, 0, 600.0, 600.0), white);
+
+  Paint clear;
+  clear.blend_mode = BlendMode::kClear;
+  clear.mask_blur_descriptor = Paint::MaskBlurDescriptor{
+      .style = FilterContents::BlurStyle::kNormal,
+      .sigma = Sigma(20),
+  };
+
+  canvas.DrawCircle(Point::MakeXY(300.0, 300.0), 200.0, clear);
+
+  ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
+}
+
+TEST_P(AiksTest, ClearBlend) {
+  Canvas canvas;
+  Paint white;
+  white.color = Color::Blue();
+  canvas.DrawRect(Rect::MakeXYWH(0, 0, 600.0, 600.0), white);
+
+  Paint clear;
+  clear.blend_mode = BlendMode::kClear;
+
+  canvas.DrawCircle(Point::MakeXY(300.0, 300.0), 200.0, clear);
+
+  ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
+}
+
 }  // namespace testing
 }  // namespace impeller

impeller/entity/contents/content_context.cc

@@ -38,10 +38,19 @@ void ContentContextOptions::ApplyToPipelineDescriptor(
 
   switch (pipeline_blend) {
     case BlendMode::kClear:
-      color0.dst_alpha_blend_factor = BlendFactor::kZero;
-      color0.dst_color_blend_factor = BlendFactor::kZero;
-      color0.src_alpha_blend_factor = BlendFactor::kZero;
-      color0.src_color_blend_factor = BlendFactor::kZero;
+      if (is_for_rrect_blur_clear) {
+        color0.alpha_blend_op = BlendOperation::kReverseSubtract;
+        color0.color_blend_op = BlendOperation::kReverseSubtract;
+        color0.dst_alpha_blend_factor = BlendFactor::kOne;
+        color0.dst_color_blend_factor = BlendFactor::kOne;
+        color0.src_alpha_blend_factor = BlendFactor::kDestinationColor;
+        color0.src_color_blend_factor = BlendFactor::kDestinationColor;
+      } else {
+        color0.dst_alpha_blend_factor = BlendFactor::kZero;
+        color0.dst_color_blend_factor = BlendFactor::kZero;
+        color0.src_alpha_blend_factor = BlendFactor::kZero;
+        color0.src_color_blend_factor = BlendFactor::kZero;
+      }
       break;
     case BlendMode::kSource:
       color0.blending_enabled = false;

impeller/entity/contents/content_context.h

@@ -308,13 +308,14 @@ struct ContentContextOptions {
   PixelFormat color_attachment_pixel_format = PixelFormat::kUnknown;
   bool has_stencil_attachment = true;
   bool wireframe = false;
+  bool is_for_rrect_blur_clear = false;
 
   struct Hash {
     constexpr std::size_t operator()(const ContentContextOptions& o) const {
-      return fml::HashCombine(o.sample_count, o.blend_mode, o.stencil_compare,
-                              o.stencil_operation, o.primitive_type,
-                              o.color_attachment_pixel_format,
-                              o.has_stencil_attachment, o.wireframe);
+      return fml::HashCombine(
+          o.sample_count, o.blend_mode, o.stencil_compare, o.stencil_operation,
+          o.primitive_type, o.color_attachment_pixel_format,
+          o.has_stencil_attachment, o.wireframe, o.is_for_rrect_blur_clear);
     }
   };
 
@@ -329,7 +330,8 @@ struct ContentContextOptions {
              lhs.color_attachment_pixel_format ==
                  rhs.color_attachment_pixel_format &&
              lhs.has_stencil_attachment == rhs.has_stencil_attachment &&
-             lhs.wireframe == rhs.wireframe;
+             lhs.wireframe == rhs.wireframe &&
+             lhs.is_for_rrect_blur_clear == rhs.is_for_rrect_blur_clear;
     }
   };
 

impeller/entity/contents/solid_rrect_blur_contents.cc

@@ -88,8 +88,13 @@ bool SolidRRectBlurContents::Render(const ContentContext& renderer,
 
   Command cmd;
   DEBUG_COMMAND_INFO(cmd, "RRect Shadow");
-  auto opts = OptionsFromPassAndEntity(pass, entity);
+  ContentContextOptions opts = OptionsFromPassAndEntity(pass, entity);
   opts.primitive_type = PrimitiveType::kTriangle;
+  Color color = color_;
+  if (entity.GetBlendMode() == BlendMode::kClear) {
+    opts.is_for_rrect_blur_clear = true;
+    color = Color::White();
+  }
   cmd.pipeline = renderer.GetRRectBlurPipeline(opts);
   cmd.stencil_reference = entity.GetStencilDepth();
 
@@ -102,7 +107,7 @@ bool SolidRRectBlurContents::Render(const ContentContext& renderer,
   VS::BindFrameInfo(cmd, pass.GetTransientsBuffer().EmplaceUniform(frame_info));
 
   FS::FragInfo frag_info;
-  frag_info.color = color_;
+  frag_info.color = color;
   frag_info.blur_sigma = blur_sigma;
   frag_info.rect_size = Point(positive_rect.size);
   frag_info.corner_radius =