frames: Use cairo renderer on GTK frames

Going for the default GL renderer is known to trigger rendering
artifacts using the NVidia proprietary driver. Since we don't have
too many expectatives about frames being flashy (not to the point
of mandating GL), resort to the cairo renderer in the mean time.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2976>

src/frames/main.c

@@ -42,6 +42,8 @@ main (int   argc,
   GMainLoop *loop;
   Display *xdisplay;
 
+  g_setenv ("GSK_RENDERER", "cairo", TRUE);
+
   /* We do know the desired GDK backend, don't let
    * anyone tell us otherwise.
    */