[gtk/wip/baedert/gl-rework: 146/176] gl renderer: Move color computation to vertex shader

[Timm Bäder <baedert src gnome org>]

commit 74b2ea8d29c7e57d96004e61b17327849b915946
Author: Timm Bäder <mail baedert org>
Date:   Tue Dec 17 16:06:28 2019 +0100

    gl renderer: Move color computation to vertex shader

 gsk/resources/glsl/border.glsl | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)
---
diff --git a/gsk/resources/glsl/border.glsl b/gsk/resources/glsl/border.glsl
index 6a0ff18024..a47fe7e393 100644
--- a/gsk/resources/glsl/border.glsl
+++ b/gsk/resources/glsl/border.glsl
@@ -1,14 +1,22 @@
 // VERTEX_SHADER:
+uniform vec4 u_color;
+
+_OUT_ vec4 final_color;
 
 void main() {
   gl_Position = u_projection * u_modelview * vec4(aPosition, 0.0, 1.0);
+
+  final_color = u_color;
+  final_color.rgb *= final_color.a; // pre-multiply
+  final_color *= u_alpha;
 }
 
 // FRAGMENT_SHADER:
-uniform vec4 u_color;
 uniform vec4 u_widths;
 uniform vec4[3] u_outline_rect;
 
+_IN_ vec4 final_color;
+
 void main() {
   vec4 f = gl_FragCoord;
   f.x += u_viewport.x;
@@ -21,9 +29,5 @@ void main() {
                        rounded_rect_coverage (inside, f.xy),
                        0.0, 1.0);
 
-  /* Pre-multiply */
-  vec4 color = u_color;
-  color.rgb *= color.a;
-
-  setOutputColor (color * alpha * u_alpha);
+  setOutputColor(final_color * alpha);
 }