[Impeller] started scaling the gaussian blur sigma to match skia output (flutter/engine#47405)

fixes https://github.com/flutter/flutter/issues/127770

This will help performance as well since our kernels will be smaller.
The justification for scaling the sigma is that we need to account in
the math the scaling down that we are doing of the input.

Here are the side-by-side comparisons with this math and skia, impeller
is always on the right:

## sigma 10

<img width="1395" alt="blur_10"
src="https://github.com/flutter/engine/assets/30870216/d534d71f-b35b-46aa-be79-8337172968a8">

## sigma 75

<img width="1398" alt="blur_75"
src="https://github.com/flutter/engine/assets/30870216/a8a77048-5212-470d-b537-a0aea98efce6">

## sigma 164 (sigma in bug)
<img width="1402" alt="blur_164"
src="https://github.com/flutter/engine/assets/30870216/cc620d02-4460-4cc8-9798-8dd21530eeed">

## sigma 400
<img width="1404" alt="blur_400"
src="https://github.com/flutter/engine/assets/30870216/653c1f1b-8156-4cff-bfcf-bf1ba737b386">

## sigma 1000
<img width="1395" alt="blur_1000"
src="https://github.com/flutter/engine/assets/30870216/c3456275-7876-4733-a27b-6022140c6a24">



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above.
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engine/src/flutter/impeller/entity/contents/filters/directional_gaussian_blur_filter_contents.cc

@@ -23,6 +23,23 @@
 
 namespace impeller {
 
+// This function was derived with polynomial regression when comparing the
+// results with Skia.  Changing the curve below should invalidate this.
+//
+// The following data points were used:
+//   0 | 1
+//  75 | 0.8
+// 150 | 0.5
+// 300 | 0.22
+// 400 | 0.2
+// 500 | 0.15
+Sigma ScaleSigma(Sigma sigma) {
+  // Limit the kernel size to 1000x1000 pixels, like Skia does.
+  Scalar clamped = std::min(sigma.sigma, 500.0f);
+  Scalar scalar = 1.02 - 3.89e-3 * clamped + 4.36e-06 * clamped * clamped;
+  return Sigma(clamped * scalar);
+}
+
 DirectionalGaussianBlurFilterContents::DirectionalGaussianBlurFilterContents() =
     default;
 
@@ -76,8 +93,7 @@ std::optional<Entity> DirectionalGaussianBlurFilterContents::RenderFilter(
     return std::nullopt;
   }
 
-  // Limit the kernel size to 1000x1000 pixels, like Skia does.
-  auto radius = std::min(Radius{blur_sigma_}.radius, 500.0f);
+  auto radius = Radius{ScaleSigma(blur_sigma_)}.radius;
 
   auto transform = entity.GetTransformation() * effect_transform.Basis();
   auto transformed_blur_radius =