window: Move frame field to WindowX11

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3254>

src/x11/window-x11-private.h

@@ -99,6 +99,8 @@ struct _MetaWindowX11Private
 
   /* maintained by group.c */
   MetaGroup *group;
+  /* may be NULL! not all windows get decorated */
+  MetaFrame *frame;
 
   gboolean has_custom_frame_extents;
   MetaSyncCounter sync_counter;
@@ -142,5 +144,8 @@ void meta_window_x11_get_session_geometry (MetaWindow  *window,
                                            int         *width,
                                            int         *height);
 
+gboolean
+meta_window_x11_get_frame_borders (MetaWindow       *window,
+                                   MetaFrameBorders *borders);
 
 G_END_DECLS