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

types/scene/drag_icon.c

@@ -18,8 +18,9 @@ static void drag_icon_handle_surface_commit(struct wl_listener *listener, void *
 	struct wlr_scene_drag_icon *icon =
 		wl_container_of(listener, icon, drag_icon_surface_commit);
 	struct wlr_surface *surface = icon->drag_icon->surface;
-	wlr_scene_node_set_position(&icon->surface_tree->node,
-		surface->sx, surface->sy);
+	struct wlr_scene_node *node = &icon->surface_tree->node;
+	wlr_scene_node_set_position(node,
+		node->x + surface->current.dx, node->y + surface->current.dy);
 }
 
 static void drag_icon_handle_map(struct wl_listener *listener, void *data) {
@@ -74,8 +75,6 @@ struct wlr_scene_tree *wlr_scene_drag_icon_create(
 		return NULL;
 	}
 
-	wlr_scene_node_set_position(&icon->surface_tree->node,
-		drag_icon->surface->sx, drag_icon->surface->sy);
 	wlr_scene_node_set_enabled(&icon->tree->node, drag_icon->mapped);
 
 	icon->tree_destroy.notify = drag_icon_handle_tree_destroy;

types/wlr_compositor.c

@@ -439,8 +439,6 @@ static void surface_commit_state(struct wlr_surface *surface,
 
 	bool invalid_buffer = next->committed & WLR_SURFACE_STATE_BUFFER;
 
-	surface->sx += next->dx;
-	surface->sy += next->dy;
 	surface_update_damage(&surface->buffer_damage, &surface->current, next);
 
 	pixman_region32_clear(&surface->external_damage);