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.