Fix #1129

wlr_drag_icon.{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.

If sx, sy were also intended for other usages or it could somehow be useful to store the buffer offset there as well (kind of redundant though), i can revert the removing from wlr and simply leave them ignored by rootston.

Tested with weston-dnd and qutebrowser (as a bit more complex dnd client).

Fixes #1129 (closed)