Want CAF with pakt in front, similar to mp4 faststart

[Mingye Wang]

Summary of the bug:

The CAF format uses a pakt table to enable VBR seeking, but ffmpeg always puts it at the end. It would be nice to have an option to put the table in front.

How to reproduce:

% ffmpeg -i airport-af.caf -c:a copy airport-ff.caf
ffmpeg version 4.4-full_build-www.gyan.dev Copyright (c) 2000-2021 the FFmpeg developers
  built with gcc 10.2.0 (Rev6, Built by MSYS2 project)

Pull out the two files (already attached) in a HEX editor, and the difference in chunk order is obvious. (I'd welcome any recommendation of a CAF structure dumper!) afconvert puts a pakt in front by default, while ffmpeg sticks it at the end.

PS: fixing this WILL NOT allow us to create VBR-Opus CAF playable on macOS. This is a shortcoming of the system, not of ffmpeg.

[Mingye Wang]

Two caf files, one fresh out of afconvert -d opus -f caff, the other from ffmpeg