sx, sy used to store the buffer offset of the drag surface which was
then be added (by rootston) to the drag icon position.
Buffer offsets are handled already in surface_intersect_output
(output.c) so they were added twice for dnd surfaces.
This lets clients bind to a seat multiple times by re-using the existing
wlr_seat_client whenever a duplicate request happens.
Previously, an independant wlr_seat_client would be created and only
events from one would be processed.
Fixes#1023.
nyorain
Drew DeVault
emersion
Tony Crisci
Dominique Martinet