• Print failing ffmpeg stderr output.
• Preserve all input file streams (e.g. audio, subs, attachments) into output.
• Support concurrent running processes out of the box by segregating temp-dirs & fixing cache access.
• Improve vmaf accuracy in some cases by forcing 24fps & synchronizing the presentation timestamp.
• Automatically workaround ffmpeg "Can't write packet with unknown timestamp" sample generation failures
  (typically encountered with old avi files) by using `-fflags +genpts`.