[mutter] compositor: Update outdated URL

[Marge Bot <marge-bot src gnome org>]

commit ff14ed5ad9f995706cce04b1f93535880fe04653
Author: Florian Müllner <fmuellner gnome org>
Date:   Wed Jul 14 20:46:33 2021 +0200

    compositor: Update outdated URL
    
    The file was moved a while ago. Update the link to the new location,
    and specify a particular tag instead of master to make sure this
    doesn't happen again.
    
    Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1930>

 src/compositor/meta-background-content.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/compositor/meta-background-content.c b/src/compositor/meta-background-content.c
index 3afeca4827..86f7777528 100644
--- a/src/compositor/meta-background-content.c
+++ b/src/compositor/meta-background-content.c
@@ -138,7 +138,7 @@ typedef enum
 /* The ellipsis_dist(), ellipsis_coverage() and rounded_rect_coverage() are
  * copied from GSK, see gsk_ellipsis_dist(), gsk_ellipsis_coverage(), and
  * gsk_rounded_rect_coverage() here:
- * https://gitlab.gnome.org/GNOME/gtk/-/blob/master/gsk/resources/glsl/preamble.fs.glsl
+ * https://gitlab.gnome.org/GNOME/gtk/-/blob/4.3.1/gsk/gl/resources/preamble.fs.glsl
  */
 #define ROUNDED_CLIP_FRAGMENT_SHADER_DECLARATIONS                            \
 "uniform vec4 bounds;           // x, y: top left; w, v: bottom right     \n"\