[Impeller] Started expanding the blur clip region (flutter#48535)

fixes flutter/flutter#139317
test: AiksTest.CanRenderClippedBlur

This works by double padding. First it applies the padding to clip
region, then it add a transparent gutter around the snapshot to account
for any of the expanded clip region that was ignored. This is suboptimal
since there maybe a considerable region for which blur is calculated
that is clipped. That can be optimized in the future by adjusting the
UVs in the blur phases to match the clip region.

## screenshot of result
<img width="1026" alt="Screenshot 2023-11-30 at 10 28 14 AM"
src="https://github.com/flutter/engine/assets/30870216/a0c673db-8b7a-465e-a75c-09f19d1b98aa">


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- [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 `///`).
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impeller/entity/contents/filters/gaussian_blur_filter_contents.cc

@@ -17,6 +17,17 @@ using GaussianBlurVertexShader = GaussianBlurPipeline::VertexShader;
 using GaussianBlurFragmentShader = GaussianBlurPipeline::FragmentShader;
 
 namespace {
+
+std::optional<Rect> ExpandCoverageHint(const std::optional<Rect>& coverage_hint,
+                                       const Matrix& source_to_local_transform,
+                                       const Vector2& padding) {
+  if (!coverage_hint.has_value()) {
+    return std::nullopt;
+  }
+  Vector2 transformed_padding = (source_to_local_transform * padding).Abs();
+  return coverage_hint->Expand(transformed_padding);
+}
+
 SamplerDescriptor MakeSamplerDescriptor(MinMagFilter filter,
                                         SamplerAddressMode address_mode) {
   SamplerDescriptor sampler_desc;
@@ -43,6 +54,8 @@ Matrix MakeAnchorScale(const Point& anchor, Vector2 scale) {
          Matrix::MakeTranslation({-anchor.x, -anchor.y, 0});
 }
 
+/// Makes a subpass that will render the scaled down input and add the
+/// transparent gutter required for the blur halo.
 std::shared_ptr<Texture> MakeDownsampleSubpass(
     const ContentContext& renderer,
     std::shared_ptr<Texture> input_texture,
@@ -66,7 +79,8 @@ std::shared_ptr<Texture> MakeDownsampleSubpass(
         frame_info.alpha = 1.0;
 
         // Insert transparent gutter around the downsampled image so the blur
-        // creates a halo effect.
+        // creates a halo effect. This compensates for when the expanded clip
+        // region can't give us the full gutter we want.
         Vector2 texture_size = Vector2(input_texture->GetSize());
         Quad vertices =
             MakeAnchorScale({0.5, 0.5},
@@ -106,6 +120,8 @@ std::shared_ptr<Texture> MakeBlurSubpass(
     std::shared_ptr<Texture> input_texture,
     const SamplerDescriptor& sampler_descriptor,
     const GaussianBlurFragmentShader::BlurInfo& blur_info) {
+  // TODO(gaaclarke): This blurs the whole image, but because we know the clip
+  //                  region we could focus on just blurring that.
   ISize subpass_size = input_texture->GetSize();
   ContentContext::SubpassCallback subpass_callback =
       [&](const ContentContext& renderer, RenderPass& pass) {
@@ -200,9 +216,23 @@ std::optional<Entity> GaussianBlurFilterContents::RenderFilter(
     return std::nullopt;
   }
 
+  Scalar blur_radius = CalculateBlurRadius(sigma_);
+  Vector2 padding(ceil(blur_radius), ceil(blur_radius));
+
+  // Apply as much of the desired padding as possible from the source. This may
+  // be ignored so must be accounted for in the downsample pass by adding a
+  // transparent gutter.
+  std::optional<Rect> expanded_coverage_hint = ExpandCoverageHint(
+      coverage_hint, entity.GetTransform() * effect_transform, padding);
+  // TODO(gaaclarke): How much of the gutter is thrown away can be used to
+  //                  adjust the padding that is added in the downsample pass.
+  //                  For example, if we get all the padding we requested from
+  //                  the expanded_coverage_hint, there is no need to add a
+  //                  transparent gutter.
+
   std::optional<Snapshot> input_snapshot =
       inputs[0]->GetSnapshot("GaussianBlur", renderer, entity,
-                             /*coverage_limit=*/coverage_hint);
+                             /*coverage_limit=*/expanded_coverage_hint);
   if (!input_snapshot.has_value()) {
     return std::nullopt;
   }
@@ -212,14 +242,11 @@ std::optional<Entity> GaussianBlurFilterContents::RenderFilter(
                                 entity.GetClipDepth());  // No blur to render.
   }
 
-  Scalar blur_radius = CalculateBlurRadius(sigma_);
   Scalar desired_scalar = CalculateScale(sigma_);
-  // TODO(jonahwilliams): if scaling value is 1.0, then skip the downsample
+  // TODO(jonahwilliams): If desired_scalar is 1.0 and we fully acquired the
+  // gutter from the expanded_coverage_hint, we can skip the downsample pass.
   // pass.
-
   Vector2 downsample_scalar(desired_scalar, desired_scalar);
-  Vector2 padding(ceil(blur_radius), ceil(blur_radius));
-
   Vector2 padded_size =
       Vector2(input_snapshot->texture->GetSize()) + 2.0 * padding;
   Vector2 downsampled_size = padded_size * downsample_scalar;