fftools/ffmpeg: move OutputStream.sq_frame to Encoder

It is audio/video encoding-only and does not need to be visible outside
of ffmpeg_enc.c

fftools/ffmpeg_mux.c

@@ -663,7 +663,6 @@ static void ost_free(OutputStream **post)
     av_bsf_free(&ms->bsf_ctx);
 
     av_frame_free(&ost->filtered_frame);
-    av_frame_free(&ost->sq_frame);
     av_packet_free(&ost->pkt);
     av_dict_free(&ost->encoder_opts);