gl renderer: Fix text color pre-multiplication

gsk/resources/glsl/coloring.fs.glsl

@@ -7,7 +7,7 @@ void main() {
   // pre-multiply
   color.rgb *= color.a;
 
-  color = vec4(u_color.rgb * diffuse.a * u_alpha, diffuse.a * color.a * u_alpha);
+  // u_source is drawn using cairo, so already pre-multiplied.
 
-  setOutputColor(color);
+  setOutputColor(color * diffuse * u_alpha);
 }