[Impeller] Turned on new blur. (flutter/engine#48472)

This new blur should perform faster since it scales down the image
before blurring it. Jonah did early testing of it and found it to be
faster. Scrolling around with the blur perf bug it seems faster. It also
has a wider test bed and is hopefully easier to maintain since it
contains all of its logic for both directions.

testing: There are existing blur tests and we've backfilled more as
we've added features to this blur.

fixes https://github.com/flutter/flutter/issues/131580
fixes https://github.com/flutter/flutter/issues/138259

## Pre-launch Checklist

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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.

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engine/src/flutter/impeller/entity/contents/filters/directional_gaussian_blur_filter_contents.h

@@ -43,6 +43,9 @@ namespace impeller {
 ///   - `FilterContents::MakeGaussianBlur`
 ///   - //flutter/impeller/entity/shaders/gaussian_blur/gaussian_blur.glsl
 ///
+///\deprecated Previously 2 of these were chained to do 2D blurs, use
+///            \ref GaussianBlurFilterContents instead since it has better
+///            performance.
 class DirectionalGaussianBlurFilterContents final : public FilterContents {
  public:
   DirectionalGaussianBlurFilterContents();

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

@@ -56,38 +56,10 @@ std::shared_ptr<FilterContents> FilterContents::MakeGaussianBlur(
     Sigma sigma_y,
     BlurStyle blur_style,
     Entity::TileMode tile_mode) {
-  constexpr bool use_new_filter =
-#ifdef IMPELLER_ENABLE_NEW_GAUSSIAN_FILTER
-      true;
-#else
-      false;
-#endif
-
-  // TODO(https://github.com/flutter/flutter/issues/131580): Remove once the new
-  // blur handles all cases.
-  if (use_new_filter) {
-    auto blur = std::make_shared<GaussianBlurFilterContents>(
-        sigma_x.sigma, sigma_y.sigma, tile_mode);
-    blur->SetInputs({input});
-    return blur;
-  }
-  std::shared_ptr<FilterContents> x_blur = MakeDirectionalGaussianBlur(
-      /*input=*/input,
-      /*sigma=*/sigma_x,
-      /*direction=*/Point(1, 0),
-      /*blur_style=*/BlurStyle::kNormal,
-      /*tile_mode=*/tile_mode,
-      /*is_second_pass=*/false,
-      /*secondary_sigma=*/{});
-  std::shared_ptr<FilterContents> y_blur = MakeDirectionalGaussianBlur(
-      /*input=*/FilterInput::Make(x_blur),
-      /*sigma=*/sigma_y,
-      /*direction=*/Point(0, 1),
-      /*blur_style=*/blur_style,
-      /*tile_mode=*/tile_mode,
-      /*is_second_pass=*/true,
-      /*secondary_sigma=*/sigma_x);
-  return y_blur;
+  auto blur = std::make_shared<GaussianBlurFilterContents>(
+      sigma_x.sigma, sigma_y.sigma, tile_mode);
+  blur->SetInputs({input});
+  return blur;
 }
 
 std::shared_ptr<FilterContents> FilterContents::MakeBorderMaskBlur(