glsl-effect: Don't overwrite the framebuffer's alpha/X channel

It's a waste of effort and will also taint screenshots if they
accidentally store that channel as alpha.

Fixes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/6389
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2650>

diff --git a/src/shell-glsl-effect.c b/src/shell-glsl-effect.c
index 3051e0af9de75469e25dc73ba796637a38a322f8..6eb6e4278532b63335ffa4fd32fbe77a386dce1c 100644 (file)
--- a/src/shell-glsl-effect.c
+++ b/src/shell-glsl-effect.c
@@ -119,7 +119,7 @@ shell_glsl_effect_constructed (GObject *object)
   if (G_UNLIKELY (klass->base_pipeline == NULL))
     {
       klass->base_pipeline = cogl_pipeline_new (ctx);
-      cogl_pipeline_set_blend (klass->base_pipeline, "RGBA = ADD (SRC_COLOR * (SRC_COLOR[A]), DST_COLOR * (1-SRC_COLOR[A]))", NULL);
+      cogl_pipeline_set_blend (klass->base_pipeline, "RGB = ADD (SRC_COLOR * (SRC_COLOR[A]), DST_COLOR * (1-SRC_COLOR[A]))", NULL);
 
       if (klass->build_pipeline != NULL)
         klass->build_pipeline (self);