Improve the LayoutTest related with Config conversions for Canvas2D

LayoutTests/canvas/philip/tests/2d.gradient.interpolate.colouralpha.html

Index: LayoutTests/canvas/philip/tests/2d.gradient.interpolate.colouralpha.html
diff --git a/LayoutTests/canvas/philip/tests/2d.gradient.interpolate.colouralpha.html b/LayoutTests/canvas/philip/tests/2d.gradient.interpolate.colouralpha.html
index ea4df4bc0a5d42fe69f8754f2e8b2d046dfe4978..ace954bd4d965cf1fb145cc30daf73717168d550 100644
--- a/LayoutTests/canvas/philip/tests/2d.gradient.interpolate.colouralpha.html
+++ b/LayoutTests/canvas/philip/tests/2d.gradient.interpolate.colouralpha.html
@@ -17,7 +17,7 @@ g.addColorStop(0, 'rgba(255,255,0, 0)');
 g.addColorStop(1, 'rgba(0,0,255, 1)');
 ctx.fillStyle = g;
 ctx.fillRect(0, 0, 100, 50);
-_assertPixelApprox(canvas, 25,25, 191,191,63,63, "25,25", "191,191,63,63", 3);
+_assertPixelApprox(canvas, 25,25, 191,191,63,63, "25,25", "191,191,63,63", 4);
 _assertPixelApprox(canvas, 50,25, 127,127,127,127, "50,25", "127,127,127,127", 3);
 _assertPixelApprox(canvas, 75,25, 63,63,191,191, "75,25", "63,63,191,191", 3);