wayland/xdg-toplevel-drag: Support non-grabbing mode in MetaWindowDrag

- This adds a new ClutterActor parameter to meta_compositor_drag_window
  and meta_window_drag_begin such that it's possible to inject an
  external grab actor, which is then used to implement a "non-grabbing"
  mode that allow a MetaWindowDrag to be used in conjunction, for
  example, with a Wayland drag-and-drop session;
- This new API is going to be used to implement the toplevel drag
  protocol in a subsequent commit.

Signed-off-by: Nick Diego Yamane <nickdiego@igalia.com>
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/4107>

src/core/window.c

@@ -6149,7 +6149,7 @@ meta_window_begin_grab_op (MetaWindow           *window,
                                       window, op,
                                       device, sequence,
                                       timestamp,
-                                      pos_hint);
+                                      pos_hint, NULL);
 }
 
 MetaStackLayer