Voice Processor (VP) multipass feature allows configuring lists of voices
that are first mixed (in order) into a designated mixbin which is then used
as a sample source when processing voices with multipass flag set to true
in NV_PAVS_VOICE_CFG_FMT. Setting correct voice order in lists is the
responsibility of the game/application and in practice is handled by the
DirectSound library. The multipass mixbin is hardcoded to 31 in
DirectSound, but hardware would allow other bins.
This implementation also adds additional info to audio debug UI to see what
the source and destination voices involved are. The info is only shown
when DSP processing is off, i.e. "VP Only" (MON_VP) is selected. This is
because storing the voice numbers requires additional digging which is
required for MON_VP anyway and therefore is free. The multipass feature
itself works fine with DSP (i.e. GP and EP) enabled, only the additional
debug info is not shown.
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