refqueue: free referenced images on free

Otherwise stale references will survive forever. Could leak hardware
video surfaces.

In particular, the mpv vdpau code crashed with an assertion when exiting
after toggling deinterlacing, because not all references were released.

video/filter/refqueue.c

@@ -47,6 +47,7 @@ struct mp_refqueue *mp_refqueue_alloc(void)
 
 void mp_refqueue_free(struct mp_refqueue *q)
 {
+    mp_refqueue_flush(q);
     talloc_free(q);
 }