fftools/ffmpeg: deprecate the -map_channel option

It is now entirely redundant with audio filters, and is in fact
implemented by setting up a 'pan' filter instance.

fftools/ffmpeg.c

@@ -578,8 +578,10 @@ static void ffmpeg_cleanup(int ret)
         av_freep(&ost->avfilter);
         av_freep(&ost->logfile_prefix);
 
+#if FFMPEG_OPT_MAP_CHANNEL
         av_freep(&ost->audio_channels_map);
         ost->audio_channels_mapped = 0;
+#endif
 
         av_dict_free(&ost->sws_dict);
         av_dict_free(&ost->swr_opts);