Add SK_USE_LEGACY_BLUR_RASTER staging flag (flutter#54314)

Skia's CPU-based blur implementation is inaccurate for sigma < 2, but
the Dart imagefilter test uses a case with sigma = 1.

In https://skia-review.googlesource.com/c/skia/+/885107, the algorithm
is updated to be more accurate when sigma < 2, so the output is more
accurate. In order to roll into flutter, the expected color values in
the tests will need to be updated with the removal of this staging flag.

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skia/flutter_defines.gni

@@ -13,6 +13,7 @@ flutter_defines = [
   "SK_LEGACY_IGNORE_DRAW_VERTICES_BLEND_WITH_NO_SHADER",
   "SK_DISABLE_LEGACY_METAL_BACKEND_SURFACE",
   "SK_DISABLE_LEGACY_PARAGRAPH_UNICODE",
+  "SK_USE_LEGACY_BLUR_RASTER",
 
   # Fast low-precision software rendering isn't a priority for Flutter.
   "SK_DISABLE_LEGACY_SHADERCONTEXT",