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.

include/backend/drm/drm.h

@@ -107,6 +107,7 @@ struct wlr_drm_backend {
 
 	/* Only initialized on multi-GPU setups */
 	struct wlr_drm_renderer mgpu_renderer;
+	struct wlr_multi_gpu *multi_gpu;
 
 	struct wlr_session *session;