Dropping files could crash ~10% of the time on some systems,
although I wasn't able to reproduce the error.
The ownership of GWL_Seat::data_offer_dnd wasn't handled correctly,
where the value could be handled by both wl_data_device_listener::leave
& drop callbacks.
Resolve by ensuring the data-offer is handled by the drop callback.
Campbell Barton
20756a07cd