xwayland: confine motion events to the confined window

When an X11 client has an active grab on the pointer, all events are
reported relative to the window with the grab.

For Xwayland, if an X11 client has a grab with a pointer confinement
active, while pointer focus is on another window, motion events should
not be reported to the client with the grab, because that sets the X11
client appart, the events would be reported when the pointer is on any
X11 window but not on Wayland native surfaces.

Therefore, if the pointer is confined on a window and that window
differs from the actual pointer focus window, just pretend we lost
pointer focus to another window.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Closes: #962


Reviewed-by: Carlos Garnacho <carlosg@gnome.org>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>