window-actor: Add a new can_freeze_commits() API

Mutter freezes Xwayland commits when resizing windows, and thaw them in
the window actors' after_paint() for X11.

Yet, after_paint() could be never called, as when a new window is mapped
while the overview is active in gnome-shell.

As a result, the content of the X11 window will remain invisible to the
overview.

Add a new window actor API to tell whether commits can be frozen. For
Wayland window actors, this always return FALSE, whereas for X11 window
actors, it checks whether the Clutter actor is mapped.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1678>

(cherry picked from commit df5a5d27)