This commit makes it possible for a single client to have multiple data devices
for the same seat. This fixes issues with Firefox.
This mainly removes wlr_data_source.offer. We make sure we create one data
offer per device. We now make the offer inert when the source is destroyed.
Fixes the second half of https://github.com/swaywm/wlroots/issues/1041
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.