xwayland: Allow pointer warp on root/None window

Of sorts, as we can't honor pointer warping across the whole root window
coordinates, peek the pointer focus in these cases.

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 c217fcb4)