A client with a pointer grab on a touch device must reject the touch when
detactivating the grab while the touch is active. However, such a rejecting
must not trigger a ButtonRelease event to be emulated and sent to the
client.
Set the grabbing listener's state to HAS_END, so we simply skip delivery to
that client.
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>