To address the black shadows that sometimes show during resize with
Xwayland, we need to update the window shape regardless of the frozen
status of the window actor.
However, plain Xorg does not need this, as resized windows do not clear
to black, so add a new vfunc to window/x11 to indicate whether or not
the backing windowing system (either plain X11 or Xwayland) would
require the shape to be always updated.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/942
4d5a86327a