Formats using custom layouts tend to be weird enough that it's a pain to add support directly in the decoder. Instead should use custom layouts and I/O streamfiles that, though a bit wordy and unwieldy at the moment, are a lot more flexible.
It's possible to set a "coding" that triggers calls to the above while codec_data is still null, when using special layouts like AAX
Requires furrybob's libatrac9, plus enabling VGM_USE_ATRAC9 in vgmstream.h/compiler flag
