backend/wayland: ensure buffers are released on shutdown

destroy_wl_buffer() is called from backend_destroy(). We need to
ensure the wlr_buffer is unlocked when we're waiting for a
wl_buffer.release event from the parent compositor.

(cherry picked from commit 378f471d29)

backend/wayland/output.c

@@ -116,6 +116,9 @@ void destroy_wl_buffer(struct wlr_wl_buffer *buffer) {
 	wl_list_remove(&buffer->buffer_destroy.link);
 	wl_list_remove(&buffer->link);
 	wl_buffer_destroy(buffer->wl_buffer);
+	if (!buffer->released) {
+		wlr_buffer_unlock(buffer->buffer);
+	}
 	free(buffer);
 }