fftools/ffmpeg_filter: add filtergraph private data

Start by moving OutputStream.filtered_frame to it, which really belongs
to the filtergraph rather than the output stream.

fftools/ffmpeg_mux.c

@@ -845,7 +845,6 @@ static void ost_free(OutputStream **post)
 
     av_bsf_free(&ms->bsf_ctx);
 
-    av_frame_free(&ost->filtered_frame);
     av_packet_free(&ost->pkt);
     av_dict_free(&ost->encoder_opts);