touch: make cancel functions take a wlr_seat_client not a surface
The surface argument is only used to obtain a wlr_seat_client and semantically makes no sense. The wl_touch.cancel event applies to all touch points and all surfaces of a client.
I decided to make the functions accept a wlr_seat_client rather than a wl_client as it is directly available in the wlr_touch_point struct and simplifies the implementation.
Edited by Isaac Freund