Fix minor mask blur issues (flutter#149)

Author: bdero

impeller/entity/contents/filters/border_mask_blur_filter_contents.cc

@@ -92,9 +92,7 @@ bool BorderMaskBlurFilterContents::RenderFilter(
 
   VS::FrameInfo frame_info;
   frame_info.mvp = Matrix::MakeOrthographic(ISize(1, 1));
-  auto scale = entity.GetTransformation().GetScale();
-  frame_info.sigma_uv = Vector2(scale.x, scale.y) *
-                        Vector2(sigma_x_.sigma, sigma_y_.sigma).Abs() /
+  frame_info.sigma_uv = Vector2(sigma_x_.sigma, sigma_y_.sigma).Abs() /
                         input_snapshot->texture->GetSize();
   frame_info.src_factor = src_color_factor_;
   frame_info.inner_blur_factor = inner_blur_factor_;

impeller/entity/shaders/border_mask_blur.frag

@@ -33,7 +33,10 @@ const float kHalfSqrtTwo = 0.70710678118;
 
 // Indefinite integral of the Gaussian function (with constant range 0->1).
 float GaussianIntegral(float x, float sigma) {
-  return 0.5 + 0.5 * erf(x * (kHalfSqrtTwo / sigma));
+  // ( 1 + erf( x * (sqrt(2) / (2 * sigma) ) ) / 2
+  // Because this sigmoid is always > 1, we remap it (n * 1.07 - 0.07)
+  // so that it always fades to zero before it reaches the blur radius.
+  return 0.535 * erf(x * (kHalfSqrtTwo / sigma)) + 0.465;
 }
 
 float BoxBlurMask(vec2 uv) {