ffmpeg_opt: Print a warning if more than one -vf/-af option was specified.

Fixes ticket #4184.

fftools/ffmpeg_opt.c

@@ -1681,6 +1681,8 @@ static OutputStream *new_video_stream(OptionsContext *o, AVFormatContext *oc, in
 
     MATCH_PER_STREAM_OPT(filter_scripts, str, ost->filters_script, oc, st);
     MATCH_PER_STREAM_OPT(filters,        str, ost->filters,        oc, st);
+    if (o->nb_filters > 1)
+        av_log(NULL, AV_LOG_ERROR, "Only '-vf %s' read, ignoring remaining -vf options: Use ',' to separate filters\n", ost->filters);
 
     if (!ost->stream_copy) {
         const char *p = NULL;
@@ -1862,6 +1864,8 @@ static OutputStream *new_audio_stream(OptionsContext *o, AVFormatContext *oc, in
 
     MATCH_PER_STREAM_OPT(filter_scripts, str, ost->filters_script, oc, st);
     MATCH_PER_STREAM_OPT(filters,        str, ost->filters,        oc, st);
+    if (o->nb_filters > 1)
+        av_log(NULL, AV_LOG_ERROR, "Only '-af %s' read, ignoring remaining -af options: Use ',' to separate filters\n", ost->filters);
 
     if (!ost->stream_copy) {
         char *sample_fmt = NULL;