compositor: drop wlr_surface.{sx,sy}

The concept of a persistent accumulated surface offset is wrong
from a protocol point-of-view. wl_surface.offset is tied to a
commit, its interpretation depends on the surface role.

For example, with the following sequence:

    wl_surface@1.offset(1, 1)
    wl_surface@1.commit()
    wl_pointer@2.set_cursor(wl_surface@1, 42, 42)

The final cursor hotspot is (42, 42): the commit which happened
before the set_cursor request has no impact on the hotspot
computation.

The wlr_output_cursor logic already uses wlr_surface.current.{dx,dy}.
wlr_scene's drag icon doesn't, update it accordingly.

include/wlr/types/wlr_compositor.h

@@ -98,10 +98,6 @@ struct wlr_surface {
 	 * or something went wrong with uploading the buffer.
 	 */
 	struct wlr_client_buffer *buffer;
-	/**
-	 * The buffer position, in surface-local units.
-	 */
-	int sx, sy;
 	/**
 	 * The last commit's buffer damage, in buffer-local coordinates. This
 	 * contains both the damage accumulated by the client via