[Impeller] Turned on new blur. (flutter#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 flutter/flutter#131580
fixes flutter/flutter#138259

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Author: gaaclarke

impeller/aiks/aiks_unittests.cc

@@ -3758,7 +3758,7 @@ TEST_P(AiksTest, GaussianBlurSetsMipCountOnPass) {
   canvas.Restore();
 
   Picture picture = canvas.EndRecordingAsPicture();
-  EXPECT_EQ(1, picture.pass->GetRequiredMipCount());
+  EXPECT_EQ(4, picture.pass->GetRequiredMipCount());
 }
 
 TEST_P(AiksTest, GaussianBlurAllocatesCorrectMipCountRenderTarget) {
@@ -3782,7 +3782,7 @@ TEST_P(AiksTest, GaussianBlurAllocatesCorrectMipCountRenderTarget) {
     max_mip_count =
         std::max(it->texture->GetTextureDescriptor().mip_count, max_mip_count);
   }
-  EXPECT_EQ(max_mip_count, 1lu);
+  EXPECT_EQ(max_mip_count, 4lu);
 }
 
 TEST_P(AiksTest, GaussianBlurMipMapNestedLayer) {
@@ -3809,7 +3809,7 @@ TEST_P(AiksTest, GaussianBlurMipMapNestedLayer) {
     max_mip_count =
         std::max(it->texture->GetTextureDescriptor().mip_count, max_mip_count);
   }
-  EXPECT_EQ(max_mip_count, 1lu);
+  EXPECT_EQ(max_mip_count, 4lu);
   EXPECT_EQ(log_capture.str().find(GaussianBlurFilterContents::kNoMipsError),
             std::string::npos);
 }

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();

impeller/entity/contents/filters/filter_contents.cc

@@ -50,50 +50,18 @@ std::shared_ptr<FilterContents> FilterContents::MakeDirectionalGaussianBlur(
   return blur;
 }
 
-#ifdef IMPELLER_ENABLE_NEW_GAUSSIAN_FILTER
 const int32_t FilterContents::kBlurFilterRequiredMipCount = 4;
-#else
-const int32_t FilterContents::kBlurFilterRequiredMipCount = 1;
-#endif
 
 std::shared_ptr<FilterContents> FilterContents::MakeGaussianBlur(
     const FilterInput::Ref& input,
     Sigma sigma_x,
     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(