Use decal TileMode in blur image_filter_test.dart (#45004)

The expected color values appear to match the output of Skia's raster
backend's blur. Historically, that doesn't support any tile mode other
than decal, so while the `makeBlur()` function defaulted to clamp
tiling, the output was decal and thus showed the blur fading to
transparent.

The test draws a 1x1 green rectangle in the center of a 3x3 image. Clamp
tiling would actually cause the output of the blur to just copy the
central green color to the remaining 8 pixels. This is the output of
Skia's GPU blur. I am working to land changes in Skia that make the
raster backend handle all tile modes, which then has it match the
existing GPU blur's behavior of a constant output for clamp tiling in
this test (so it then fails).

Decal tiling appears to be more useful for this test case anyways
because it creates per-pixel variations that can be validated against.

This is needed to land Skia-side fixes for skbug.com/40039877 and
skbug.com/40039025

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Author: lhkbob

testing/dart/image_filter_test.dart

@@ -176,7 +176,7 @@ void main() {
   test('ImageFilter - blur', () async {
     final Paint paint = Paint()
       ..color = green
-      ..imageFilter = makeBlur(1.0, 1.0);
+      ..imageFilter = makeBlur(1.0, 1.0, TileMode.decal);
 
     final Uint32List bytes = await getBytesForPaint(paint);
     checkBytes(bytes, greenCenterBlurred, greenSideBlurred, greenCornerBlurred);