Support loop points in MP3 BGM

[Splendide-Imaginarius]

MP3 should obviously not be used for new BGM assets. However, sometimes the master audio is unavailable, and only an MP3 version can be found. In this case, if loop points are desired, there are 3 options:

1. Transcode to a lossy format that supports loop points, needlessly damaging quality.
2. Transcode to a lossless format that supports loop points, needlessly wasting storage.
3. Add loop points to the MP3.

Obviously option 3 seems attractive. When VX Ace was under development, MP3 typically used ID3 tags, which are not suitable for loop points. This, combined with MP3 being a generally bad format for other reasons, is probably why Enterbrain didn't support loop points in MP3 files.

However, things have changed since 2011. MP3's now can have APE tags, which (unlike ID3 tags) are suitable for loop points. I was able to add loop points to an MP3 file's APE tag with kid3, and confirmed that the loop points were readable in VLC's metadata viewer.

It looks like this is not done very much in the wild, as evidenced by the fact that ffprobe and vgmstream don't seem to recognize MP3 APE loop points (unless I screwed up when testing?), and I can't find any bugs open with either project about this. (I didn't explicitly test SDL_mixer but I don't see any code there for this either, maybe I missed something.) However, "thing X isn't done much in the wild" does not imply "we shouldn't do thing X".

So, open question: is there any reason we shouldn't support APE loop points in MP3 BGM?

[Splendide-Imaginarius]

Based on Matrix/Discord discussion, it sounds like this is not an unreasonable feature to add, but also will probably not benefit a vast number of users. I will probably try to implement this in the foreseeable future.