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.