output: make rollback_render mandatory

If the output backend provides attach_render, assert it also provides a
way to revert it via rollback_render.

types/wlr_output.c

@@ -314,7 +314,7 @@ static void handle_display_destroy(struct wl_listener *listener, void *data) {
 
 void wlr_output_init(struct wlr_output *output, struct wlr_backend *backend,
 		const struct wlr_output_impl *impl, struct wl_display *display) {
-	assert(impl->attach_render && impl->commit);
+	assert(impl->attach_render && impl->rollback_render && impl->commit);
 	if (impl->set_cursor || impl->move_cursor) {
 		assert(impl->set_cursor && impl->move_cursor);
 	}