Revert "Temporarily disable dither on gradients, so we can update WR."

This reverts commit 8d46372.

webrender/res/ps_gradient.fs.glsl

@@ -12,9 +12,5 @@ void main(void) {
 #endif
 
     alpha = min(alpha, do_clip());
-
-    // TODO(gw): Re-enable the gradient dither once we get the
-    //           reftests passing.
-    //oFragColor = dither(vColor * vec4(1.0, 1.0, 1.0, alpha));
-    oFragColor = vColor * vec4(1.0, 1.0, 1.0, alpha);
+    oFragColor = dither(vColor * vec4(1.0, 1.0, 1.0, alpha));
 }

wrench/reftests/gradient/reftest.list

@@ -2,9 +2,9 @@
 == premultiplied-angle.yaml blank.yaml
 == premultiplied-radial.yaml blank.yaml
 
-fuzzy(1,20000) == linear.yaml linear-ref.png
-fuzzy(1,20000) == linear-reverse.yaml linear-ref.png
-fuzzy(1,29300) == linear-stops.yaml linear-stops-ref.png
+== linear.yaml linear-ref.png
+== linear-reverse.yaml linear-ref.png
+fuzzy(1,300) == linear-stops.yaml linear-stops-ref.png
 
 # dithering requires us to fuzz here
 fuzzy(1,20000) == linear.yaml linear-ref.yaml