
When a drag was performed but did not finish successfully, e.g. because no
mimetype was accepted, we need to send the source cancel event so clients
know they can destroy the drag source (since version 3).
Fixes https://gitlab.gnome.org/GNOME/mutter/issues/740
https://gitlab.gnome.org/GNOME/mutter/merge_requests/1031
(cherry picked from commit 6cc748cce9
)