window: Ignoring unmanaging ancestor when finding root

This avoids the following critical warning happening sometimes when a
Wayland client exits taking all its window with it in an arbitrary
order:

    CRITICAL: meta_window_set_stack_position_no_sync: assertion 'window->stack_position >= 0' failed

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3870>

src/core/window.c

@@ -4946,6 +4946,7 @@ find_root_ancestor (MetaWindow *window,
   MetaWindow **ancestor = data;
 
   /* Overwrite the previously "most-root" ancestor with the new one found */
+  if (!window->unmanaging)
     *ancestor = window;
 
   /* We want this to continue until meta_window_foreach_ancestor quits because