[gtk] gl renderer: Properly apply opacity in cross-fade shader

[Timm Bäder <baedert src gnome org>]

commit 34fcfb154ab2e0cd1d51120a9b11a83b4868c9ad
Author: Timm Bäder <mail baedert org>
Date:   Wed Apr 24 12:56:23 2019 +0200

    gl renderer: Properly apply opacity in cross-fade shader

 gsk/resources/glsl/cross_fade.fs.glsl | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)
---
diff --git a/gsk/resources/glsl/cross_fade.fs.glsl b/gsk/resources/glsl/cross_fade.fs.glsl
index 8f0f4115f4..612673bf66 100644
--- a/gsk/resources/glsl/cross_fade.fs.glsl
+++ b/gsk/resources/glsl/cross_fade.fs.glsl
@@ -6,7 +6,8 @@ void main() {
   vec4 source1 = Texture(u_source, vUv);  // start child
   vec4 source2 = Texture(u_source2, vUv); // end child
 
-  float p = u_progress;
-  vec4 color = ((1.0 - p) * source1) + (p * source2);
+  float p_start = (1.0 - u_progress) * u_alpha;
+  float p_end = u_progress * u_alpha;
+  vec4 color = (p_start * source1) + (p_end * source2);
   setOutputColor(color);
 }