video: fix midstream video configuration changes

Commit 771a8bf5 added code to avoid unnecessary vf_reconfig() calls for
unrelated reasons, but forget to consider that it has to be called at
least once if the input format changes. As a consequence it got "stuck"
due to not being able to decode more frames.

player/video.c

@@ -624,6 +624,7 @@ static int video_filter(struct MPContext *mpctx, bool eof)
             return VD_PROGRESS;
 
         // The filter chain is drained; execute the filter format change.
+        vf->initialized = 0;
         filter_reconfig(mpctx, mpctx->vo_chain);
 
         mp_notify(mpctx, MPV_EVENT_VIDEO_RECONFIG, NULL);