Avoid super-luminous pixels

The color-matrix shader was creating pixels with r,g,b > a in
some cases, which leads to unexpected test failures. In particular
this as visible the opacity render node test for opacity 0.

gsk/resources/vulkan/color-matrix.frag

@@ -22,8 +22,7 @@ color_matrix (vec4 color, mat4 color_matrix, vec4 color_offset)
   color = clamp(color, 0.0, 1.0);
 
   /* premultiply */
-  if (color.a != 0.0)
-    color.rgb *= color.a;
+  color.rgb *= color.a;
 
   return color;
 }