output: listen to renderer destroy in wlr_output_cursor

When a renderer is destroyed, so are all textures. Reset the
texture in that case.

include/wlr/types/wlr_output.h

@@ -45,6 +45,7 @@ struct wlr_output_cursor {
 	int32_t hotspot_x, hotspot_y;
 	struct wlr_texture *texture;
 	bool own_texture;
+	struct wl_listener renderer_destroy;
 	struct wl_list link;
 };