enables 2-pass rate control
1 - first pass (creates stat-file)
2 - second pass (uses stat-file to adjust bitrate)
filename of stat-file to readfrom/writeto (depends on "--pass")
Goal: improve bitrate distribution (improves quality) AND improve output file size precision: output file size should not exceed <bitrate>*<length(seconds)>/8 bytes (where <bitrate> is from "-b" option). AAC header(s) and mp4 container size should be taken in account.