Add new struct wlr_multi_gpu

For development to continue on systems (such as optimus laptops) that
have multiple GPUs, we need a way to reference all of the renderers
that have been created for devices in the system. A wlr_multi_gpu
struct holds renderers coming from two different sources:
1. the primary renderer (given to us by the compositor)
2. drm sub backends, each of which has a renderer created for
   cross-GPU copies.

This change provides a way to access all of these from the same
place.

backend/drm/renderer.c

@@ -3,6 +3,7 @@
 #include <wlr/render/swapchain.h>
 #include <wlr/render/wlr_renderer.h>
 #include <wlr/util/log.h>
+#include "backend/backend.h"
 #include "backend/drm/drm.h"
 #include "backend/drm/fb.h"
 #include "backend/drm/renderer.h"