[Impeller] Correct coverage hint usage in PipelineBlend (flutter#43708)

Resolves flutter/flutter#130411.

The remaining difference in appearance is due to flutter/flutter#127770 -- Impeller's blurs skew too large.

Before:

![Screen Shot 2023-07-14 at 3 36 02 PM](https://github.com/flutter/engine/assets/919017/54ad62ed-f091-4489-a346-a0e7580ed2f0)

After:

![Screen Shot 2023-07-14 at 3 22 25 PM](https://github.com/flutter/engine/assets/919017/252ac8bb-4a09-4c96-a405-5ddbb3ba6851)

impeller/aiks/aiks_unittests.cc

@@ -2984,6 +2984,27 @@ TEST_P(AiksTest, DrawScaledTextWithPerspectiveSaveLayer) {
 
   ASSERT_TRUE(RenderTextInCanvas(GetContext(), canvas, "Hello world",
                                  "Roboto-Regular.ttf"));
+}
+
+TEST_P(AiksTest, PipelineBlendSingleParameter) {
+  Canvas canvas;
+
+  // Should render a green square in the middle of a blue circle.
+  canvas.SaveLayer({});
+  {
+    canvas.Translate(Point(100, 100));
+    canvas.DrawCircle(Point(200, 200), 200, {.color = Color::Blue()});
+    canvas.ClipRect(Rect(100, 100, 200, 200));
+    canvas.DrawCircle(
+        Point(200, 200), 200,
+        {.color = Color::Green(),
+         .blend_mode = BlendMode::kSourceOver,
+         .image_filter = [](const FilterInput::Ref& input,
+                            const Matrix& effect_transform, bool is_subpass) {
+           return ColorFilterContents::MakeBlend(BlendMode::kSource, {input});
+         }});
+    canvas.Restore();
+  }
 
   ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
 }

impeller/entity/contents/filters/blend_filter_contents.cc

@@ -91,6 +91,21 @@ static std::optional<Entity> AdvancedBlend(
     src_uvs = maybe_src_uvs.value();
   }
 
+  Rect subpass_coverage = coverage;
+  if (entity.GetContents()) {
+    auto coverage_hint = entity.GetContents()->GetCoverageHint();
+
+    if (coverage_hint.has_value()) {
+      auto maybe_subpass_coverage =
+          subpass_coverage.Intersection(*coverage_hint);
+      if (!maybe_subpass_coverage.has_value()) {
+        return std::nullopt;  // Nothing to render.
+      }
+
+      subpass_coverage = *maybe_subpass_coverage;
+    }
+  }
+
   //----------------------------------------------------------------------------
   /// Render to texture.
   ///
@@ -159,7 +174,9 @@ static std::optional<Entity> AdvancedBlend(
     auto blend_uniform = host_buffer.EmplaceUniform(blend_info);
     FS::BindBlendInfo(cmd, blend_uniform);
 
-    frame_info.mvp = Matrix::MakeOrthographic(size);
+    frame_info.mvp =
+        Matrix::MakeOrthographic(size) *
+        Matrix::MakeTranslation(coverage.origin - subpass_coverage.origin);
 
     auto uniform_view = host_buffer.EmplaceUniform(frame_info);
     VS::BindFrameInfo(cmd, uniform_view);
@@ -168,22 +185,15 @@ static std::optional<Entity> AdvancedBlend(
     return true;
   };
 
-  auto subpass_size = ISize(coverage.size);
-  if (entity.GetContents()) {
-    auto coverage_hint = entity.GetContents()->GetCoverageHint();
-    if (coverage_hint.has_value()) {
-      subpass_size = subpass_size.Min(ISize(coverage_hint->size));
-    }
-  }
-  auto out_texture =
-      renderer.MakeSubpass("Advanced Blend Filter", subpass_size, callback);
+  auto out_texture = renderer.MakeSubpass(
+      "Advanced Blend Filter", ISize(subpass_coverage.size), callback);
   if (!out_texture) {
     return std::nullopt;
   }
 
   return Entity::FromSnapshot(
       Snapshot{.texture = out_texture,
-               .transform = Matrix::MakeTranslation(coverage.origin),
+               .transform = Matrix::MakeTranslation(subpass_coverage.origin),
                // Since we absorbed the transform of the inputs and used the
                // respective snapshot sampling modes when blending, pass on
                // the default NN clamp sampler.
@@ -498,7 +508,22 @@ static std::optional<Entity> PipelineBlend(
   auto dst_snapshot =
       inputs[0]->GetSnapshot("PipelineBlend(Dst)", renderer, entity);
   if (!dst_snapshot.has_value()) {
-    return std::nullopt;
+    return std::nullopt;  // Nothing to render.
+  }
+
+  Rect subpass_coverage = coverage;
+  if (entity.GetContents()) {
+    auto coverage_hint = entity.GetContents()->GetCoverageHint();
+
+    if (coverage_hint.has_value()) {
+      auto maybe_subpass_coverage =
+          subpass_coverage.Intersection(*coverage_hint);
+      if (!maybe_subpass_coverage.has_value()) {
+        return std::nullopt;  // Nothing to render.
+      }
+
+      subpass_coverage = *maybe_subpass_coverage;
+    }
   }
 
   ContentContext::SubpassCallback callback = [&](const ContentContext& renderer,
@@ -538,8 +563,7 @@ static std::optional<Entity> PipelineBlend(
 
       VS::FrameInfo frame_info;
       frame_info.mvp = Matrix::MakeOrthographic(pass.GetRenderTargetSize()) *
-                       Matrix::MakeTranslation(
-                           -input->GetCoverage().value_or(coverage).origin) *
+                       Matrix::MakeTranslation(-subpass_coverage.origin) *
                        input->transform;
       frame_info.texture_sampler_y_coord_scale =
           input->texture->GetYCoordScale();
@@ -595,23 +619,16 @@ static std::optional<Entity> PipelineBlend(
     return true;
   };
 
-  auto subpass_size = ISize(coverage.size);
-  if (entity.GetContents()) {
-    auto coverage_hint = entity.GetContents()->GetCoverageHint();
-    if (coverage_hint.has_value()) {
-      subpass_size = subpass_size.Min(ISize(coverage_hint->size));
-    }
-  }
-  auto out_texture =
-      renderer.MakeSubpass("Pipeline Blend Filter", subpass_size, callback);
+  auto out_texture = renderer.MakeSubpass(
+      "Pipeline Blend Filter", ISize(subpass_coverage.size), callback);
 
   if (!out_texture) {
     return std::nullopt;
   }
 
   return Entity::FromSnapshot(
       Snapshot{.texture = out_texture,
-               .transform = Matrix::MakeTranslation(coverage.origin),
+               .transform = Matrix::MakeTranslation(subpass_coverage.origin),
                // Since we absorbed the transform of the inputs and used the
                // respective snapshot sampling modes when blending, pass on
                // the default NN clamp sampler.