vulkan: Fix mask shader to premultiply color

The inColor wasn't premultiplied, so the resulting colors were wrong
with translucent borders.

gsk/resources/vulkan/mask.frag

@@ -12,5 +12,5 @@ layout(location = 0) out vec4 color;
 
 void main()
 {
-  color = clip (inPos, vec4(inColor.rgb, texture(inTexture, inTexCoord).a));
+  color = clip (inPos, vec4(inColor.rgb * inColor.a, inColor.a) * texture(inTexture, inTexCoord).a);
 }