Rename Voice's NumChannels to OutChannels
Remove an unused macro
Rework HRTF decision logic

This way takes into account a new stereo-mode config option, which when set to
"headphones" will default to using HRTF. Eventually the device will also be
able to specify if headphones are being used.
Document the stereo-mode config option
Update some config descriptions
Attempt to use BS2B when using headphones without HRTF
Remove a couple unnecessary traces
Handle loopback devices when deciding to use HRTF
Partially revert "Use a different method for HRTF mixing"

The sound localization with virtual channel mixing was just too poor, so while
it's more costly to do per-source HRTF mixing, it's unavoidable if you want
good localization.

This is only partially reverted because having the virtual channel is still
beneficial, particularly with B-Format rendering and effect mixing which
otherwise skip HRTF processing. As before, the number of virtual channels can
potentially be customized, specifying more or less channels depending on the
system's needs.
Add an option for a simpler virtual channel setup

With HRTF mixing, certain things are mixed to virtual channels to be filtered
with HRTF later. This allows for using an 8-channel cube instead of a 14-
channel cube+diamond.
Remove the cube+diamond virtual layout