fftools/ffmpeg_filter: move most of -apad logic to the muxer

The decision whether -apad actually does anything is made based on muxer
properties, and so more properly belongs there. Filtering code only
receives the result.

fftools/ffmpeg_mux.c

@@ -815,7 +815,6 @@ static void ost_free(OutputStream **post)
     av_expr_free(ost->kf.pexpr);
 
     av_freep(&ost->logfile_prefix);
-    av_freep(&ost->apad);
 
     av_freep(&ost->attachment_filename);