workspace: Only consider windows that should be showing as focusable

When selecting the default focus window, is_focusable() was not
considering the new conditions for whether a window should be shown or
hidden that were added to meta_window_should_be_showing() in 39942974.
As a result the default focus window could end up a window already
hidden or hidden once meta_window_flush_calc_showing() is called by
meta_window_focus() when focusing the default window. This would cause
meta_window_focus() to fail, which is an issue if it prevents us from
unfocusing a window when it is getting unmanaged.

Fixes: 399429742 ("x11: Integrate frames client into Mutter")
Closes: https://gitlab.gnome.org/GNOME/mutter/-/issues/2644
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2962>

src/core/workspace.c

@@ -1436,8 +1436,7 @@ is_focusable (MetaWindow    *window,
          window->unmaps_pending == 0 &&
          window->type != META_WINDOW_DOCK &&
          meta_window_is_focusable (window) &&
-         meta_window_located_on_workspace (window, workspace) &&
-         meta_window_showing_on_its_workspace (window);
+         meta_window_should_be_showing_on_workspace (window, workspace);
 }
 
 static gboolean