It will be useless to check the source window on the destination side,
it's at the moment always NULL. Fetch the display from the device instead,
which will be set for every GdkDragContext.
On wayland the DnD surface must be created early when starting the drag
operation, so offer API for GTK+ to get the GdkWindow used as a DnD
surface on the drag operation.
https://bugzilla.gnome.org/show_bug.cgi?id=697855
The wl_data_source is retrieved from the selection object for the DnD
selection, and used to initiate a drag. When the drag is finished, a
button release or touch end event is synthesized to finish the DnD
operation after the compositor grab is gone.
https://bugzilla.gnome.org/show_bug.cgi?id=697855
