backend/headless: handle renderer destroy

When the headless backend uses an already-existing renderer, it doesn't
have ownership over the renderer. When the renderer is destroyed, the
headless backend needs to destroy itself.

include/backend/headless.h

@@ -17,6 +17,7 @@ struct wlr_headless_backend {
 	size_t last_output_num;
 	struct wl_list input_devices;
 	struct wl_listener display_destroy;
+	struct wl_listener renderer_destroy;
 	bool started;
 	GLenum internal_format;
 };