Upgrade to wlroots 0.15

- Update wlr_box includes to util/box.h: the wlroots header has
  been moved upstream.
- Subsurface fields have been moved
- Create renderer and allocator, stop using wlr_backend_get_renderer
- Initalize output rendering

server.h

@@ -25,6 +25,8 @@ struct cg_server {
 	struct wl_display *wl_display;
 	struct wl_list views;
 	struct wlr_backend *backend;
+	struct wlr_renderer *renderer;
+	struct wlr_allocator *allocator;
 
 	struct cg_seat *seat;
 	struct wlr_idle *idle;