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

util.c

@@ -6,8 +6,6 @@
  * See the LICENSE file accompanying this file.
  */
 
-#include <wlr/types/wlr_box.h>
-
 #include "util.h"
 
 int