[FR] revisit MPEG-4 DRC / MPEG-D DRC

[nift4]

Currently, it's hard disabled by me in my media3 fork: https://github.com/nift4/media/commit/204c906082c744dd6a3f8a65e8170ac78eeea456

That's because only MPEG-D DRC seems to be usable for serious music purposes (it has a album mode). However, the only codec MPEG-D DRC is used with, xHE-AAC, only has a single generally available encoder (exhale) which does not support album mode. So I'm pretty sure usage is nonexistent.

But if someone does get their hands at such a file, currently it would apply both ReplayGain/SoundCheck/etc and MPEG DRC, which is not what's supposed to happen, so I disabled it for now.

To re-enable it (even if it doesn't support album mode, SoundCheck doesn't either), the target level should be corrected to -18 LUFS (instead of -16 LUFS default), DRC vs gain reduction should be decided as per the toggle, it should honor the master RG toggle, if file somehow provides it album mode should be used as appropriate, and it should be disabled if we use ReplayGain instead.

But that's low priority if there's no actual usage ATM.