[Skia] Fix failing layout tests related to blurred shadows

• Component: WPE WebKit
• Status: NEW ---

[Pawel Lampe]

Failing test cases:

fast/canvas/canvas-scale-shadowBlur.html
fast/canvas/canvas-strokePath-gradient-shadow.html
fast/canvas/canvas-strokeRect-alpha-shadow.html
fast/canvas/canvas-strokeRect-gradient-shadow.html

The above tests can be aligned this way:

diff --git a/LayoutTests/fast/canvas/canvas-strokeRect-gradient-shadow.html b/LayoutTests/fast/canvas/canvas-strokeRect-gradient-shadow.html
index 1aec50b1a419..0c19c84a4463 100644
--- a/LayoutTests/fast/canvas/canvas-strokeRect-gradient-shadow.html
+++ b/LayoutTests/fast/canvas/canvas-strokeRect-gradient-shadow.html
@@ -101,7 +102,7 @@ test(shouldBeAround, 300, 150, 255, 0, 0, 64);
 
 test(shouldBe, 400, 25,  0, 0, 0, 0);
 test(shouldBe, 525, 150, 0, 0, 0, 0);
-test(shouldBe, 400, 275, 0, 0, 0, 0);
+test(shouldBe, 400, 274, 0, 0, 0, 0);
 test(shouldBe, 275, 150, 0, 0, 0, 0);
 
 print(' ');
@@ -109,8 +110,8 @@ print('Verifying blurry shadow...');
 test(shouldBe, 400, 400, 0, 0, 0, 0);
 
 test(shouldBe, 400, 325, 0, 0, 0, 0);
-test(shouldBe, 475, 400, 0, 0, 0, 0);
-test(shouldBe, 400, 475, 0, 0, 0, 0);
+test(shouldBe, 474, 400, 0, 0, 0, 0);
+test(shouldBe, 400, 474, 0, 0, 0, 0);
 test(shouldBe, 325, 400, 0, 0, 0, 0);
 
 test(shouldBeAround, 400, 300, 255, 0, 0, 64);
@@ -119,7 +120,7 @@ test(shouldBeAround, 300, 400, 255, 0, 0, 64);
 test(shouldBeAround, 500, 400, 255, 0, 0, 64);
 
 test(shouldBe, 525, 400, 0, 0, 0, 0);
-test(shouldBe, 275, 400, 0, 0, 0, 0);
+test(shouldBe, 274, 400, 0, 0, 0, 0);
 
 print(' ');
 print('Verifying rotated alpha shadow...');

since in skia, the blur looks to be reaching 1px further than in cairo/CG. However, we need to make sure the https://www.w3.org/TR/css-backgrounds-3/#shadow-blur won't be invalidated after the tests are re-worked.