backend: automatically create renderer

If a backend accepts buffers (as indicated by get_buffer_caps) but
doesn't implement get_renderer, automatically create a renderer.

include/wlr/backend.h

@@ -25,6 +25,10 @@ struct wlr_backend {
 		/** Raised when new outputs are added, passed the wlr_output */
 		struct wl_signal new_output;
 	} events;
+
+	// Private state
+
+	struct wlr_renderer *renderer;
 };
 
 /**