Add multiple buffering to xwl_window
Xwayland takes care of not attaching a new buffer if a frame callback is pending.
Yet, the existing buffer (which was previously attached) may still be updated from the X11 side, causing unexpected visual glitches to the buffer.
Add multiple buffering to the xwl_window and alternate between buffers, to leave the Wayland buffer untouched between frame callbacks and avoid stuttering or tearing issues.
Closes: #835 (closed) Signed-off-by: Olivier Fourdan ofourdan@redhat.com