The layout was designed to do subframe deinterleave (when
less-than-a-frame bytes of each channel are interleaved) in an array and
pass it to "mem" decoders.
In practice this only happens in a handful of formats, was only used
with DSP, and since making "mem" decoders is required it's simpler to
make normal decoders handling the byte layout directly.
In an effort to simplify vgmstream's layouts, code, and other esoteric
features I changed the old decode_ngc_dsp_mem for decode_ngc_dsp_subint;
results are byte-exact.
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