[gtk] gl renderer: Remove unused variable from shader

[Timm Bäder <baedert src gnome org>]

commit d0d2ad9f5b5940f5fe5390bccaaa30dc1b4bf7d0
Author: Timm Bäder <mail baedert org>
Date:   Tue Dec 17 16:13:12 2019 +0100

    gl renderer: Remove unused variable from shader

 gsk/resources/glsl/color_matrix.glsl | 5 +----
 1 file changed, 1 insertion(+), 4 deletions(-)
---
diff --git a/gsk/resources/glsl/color_matrix.glsl b/gsk/resources/glsl/color_matrix.glsl
index bd621432fd..adaee80abc 100644
--- a/gsk/resources/glsl/color_matrix.glsl
+++ b/gsk/resources/glsl/color_matrix.glsl
@@ -10,10 +10,7 @@ uniform mat4 u_color_matrix;
 uniform vec4 u_color_offset;
 
 void main() {
-  vec4 diffuse = Texture(u_source, vUv);
-  vec4 color;
-
-  color = diffuse;
+  vec4 color = Texture(u_source, vUv);
 
   // Un-premultilpy
   if (color.a != 0.0)