x11: Wait synchronously for the frames client to exit

This does nothing wrt making race conditions shorter in the
X11 window manager switch case, but is a nice to have in order
to ensure an orderly shutdown of X11 stuff.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2796>

src/x11/meta-x11-display.c

@@ -144,6 +144,8 @@ meta_x11_display_dispose (GObject *object)
   if (x11_display->frames_client)
     {
       g_subprocess_send_signal (x11_display->frames_client, SIGTERM);
+      if (x11_display->display->closing)
+        g_subprocess_wait (x11_display->frames_client, NULL, NULL);
       g_clear_object (&x11_display->frames_client);
     }