xwayland: Fix LeaveNotify for relative pointer

Since commit 20c78f38, we use the relative pointer for enter/leave
events.

However, sprite_check_lost_focus() which verifies whether the pointer has
left an Xwayland surface still explicitly check for the absolute
pointer.

As a result, no LeaveNotify event is emitted anymore now when the
pointer crosses from an Xwayland surface to a Wayland native one.

Make sure to check the last slave device against get_pointer_event() as
well, not just the absolute pointer.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Acked-by: Martin Peres <martin.peres@mupuf.org>
Acked-by: Roman Gilg <subdiff@gmail.com>
Tested-by: Roman Gilg <subdiff@gmail.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Fixes: 20c78f38 - xwayland: use get_pointer_device() for enter/leave
                  handling too
(cherry picked from commit d7391f38)
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