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.h

@@ -1,7 +1,7 @@
 #ifndef CG_UTIL_H
 #define CG_UTIL_H
 
-#include <wlr/types/wlr_box.h>
+#include <wlr/util/box.h>
 
 /** Apply scale to a width or height. */
 int scale_length(int length, int offset, float scale);