window-actor/x11: Cache the frame bounds

When resizing an X11 window with client side decorations, the shadow is
clipped by the frame bounds so that we don't need to paint the shadow
under the opaque areas covered by the window and its frame.

When the X11 client uses the EMWH synchronization mechanism (like all
gtk-3 based clients), the actual window may not be updated so that the
actual window and it frame may be behind the expected window frame
bounds, which gives the impression of de-synchronized shadows.

To avoid the issue, keep a copy of the frame bounds as a cache and only
update it when the client is not frozen so that the clipping occurs on
the actual content.

Closes: https://gitlab.gnome.org/GNOME/mutter/issues/1178
https://gitlab.gnome.org/GNOME/mutter/merge_requests/1214

(cherry picked from commit bd45a00f)