Since the source doesn't always come from a client, this field
doesn't make sense. It is replaced by a new "finalized" field in
wlr_client_data_source. This is used to make sure set_actions is
not sent after start_drag has been sent.
A check in data_offer_choose_action has been removed: if an offer
has been sent then start_drag has been called, no need to check.
I also wanted to add a check for wl_data_source.offer, but it
turns out (1) this isn't in the spec (2) it breaks GTK+.
This is some preliminary work for Firefox on Wayland compatibility.
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.
