xwayland: Lock the pointer if it is confined and has no cursor

In the typical pattern in games of "hide cursor, grab with a confineTo,
warp constantly the pointer to the middle of the window" the last warping
step is actually rather optional. Some games may choose to just set up a
grab with confineTo argument, and trust that they'll get correct relative
X/Y axis values despite the hidden cursor hitting the confinement window
edge.

To cater for these cases, lock the pointer whenever there is a pointer
confinement and the cursor is hidden. This ensures the pointer position
is in sync with the compositor's when it's next shown again, and more
importantly resorts to the relative pointer for event delivery.

Signed-off-by: Carlos Garnacho <carlosg@gnome.org>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
(cherry picked from commit ca17f3e9)