xwayland: Pointer contraints and locking fixes

Basically, if an X11 client issues a grab while the pointer is on another X11 window, we may end up locking that pointer on that other window.

Yet, button clicks are first processed by the Wayland compositor and can cause a focus change which is is unexpected while that occurs playing a game fullscreen on another output.

The same issue occurs with plain Wayland clients, but X11 clients cannot hide the cursor on plain Wayland surface while they can on X11 windows, making the issue more confusing.

The goal here is not to fix that behaviour, it's out of the scope of Xwayland, but instead to avoid locking the pointer on another X11 window, so that the user can still move hte pointer back within the game window and the confinement can take effect.

#962 (closed)