The surface offset allows an application to move itself in relative
coordinates to its previous position. It is rather ill defined and
partly incompatible with other functionality, which is why we ignore
it generally.
For dnd-surfaces though, it is the de-facto standard for applications
to properly position the dnd-icon below the cursor. Therefore apply
the offset on actor sync by setting the feedback actor anchor.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/684
