stage-view: Bump fake swap ready notify callback priority

This will become important when dispatching happens from a frame clock
driver, where the driver itself triggers from a GSource. If the driver
source has a higher priority, we'll risk dispatching before the frame
clock has returned to a state expecting to be dispatched.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4065>

src/backends/meta-stage-view.c

@@ -218,8 +218,12 @@ meta_stage_view_perform_fake_swap (MetaStageView *view,
   };
 
   g_warn_if_fail (priv->notify_presented_handle_id == 0);
+
+  /* The priority needs to be higher than the source dispatching the frame
+   * clock, to avoid racing with it.
+   */
   priv->notify_presented_handle_id =
-    g_idle_add_full (G_PRIORITY_DEFAULT,
+    g_idle_add_full (G_PRIORITY_HIGH,
                      notify_presented_idle,
                      closure, g_free);
 }