[gtk+] gl renderer: Fix text color pre-multiplication

[Timm Bäder <baedert src gnome org>]

commit f729cbb7602426ad2ee0e0c8baf810cc1dd73e16
Author: Timm Bäder <mail baedert org>
Date:   Thu Jan 4 11:41:51 2018 +0100

    gl renderer: Fix text color pre-multiplication

 gsk/resources/glsl/coloring.fs.glsl |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/gsk/resources/glsl/coloring.fs.glsl b/gsk/resources/glsl/coloring.fs.glsl
index 9faab8c..7b2e68b 100644
--- a/gsk/resources/glsl/coloring.fs.glsl
+++ b/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);
 }