investigate adding metadata to RIFF header of mixdown wav files

[maks]

Per request from user "jsizzle2221" on discord asking to add bpm to mixdown renders, we could potentially include other metadata based on what RIFF fields are available and commonly supported.

[maks]

On initial investigation there seems to little to no standardisation of this amongst DJ software and that instead of RIFF chunks they embed ID3 tags.

For example for BPM we have that Serato, Rekordbox, Traktor have moved toward embedding an ID3v2 tag inside the WAV file as a RIFF chunk with the ID id3 (lowercase, note the trailing space).

Field	ID3 Tag	Description
BPM	TBPM	Stored as a string (e.g., "124").
Key	TKEY	Musical key (e.g., "Am" or "8A").
Title	TIT2	Standard track title.

so ID3 seems to be the way to go for this.

note for implementation: RIFF chunks must be word-aligned (even byte counts). If a chunk's data is an odd number of bytes, you must add a null padding byte at the end.

For thing like cue points it seems at least Traktor, Virtual DJ and Mixxx all support RIFF cue to some extent so could look at implementing support for that in future as a separate issue too