Cleanup some HRTF code

Use loops instead of duplicating code, rewrite some lines to be clearer.
Fix typo (overriden -> overridden)
Properly scale the first HRTF pass-through coefficient

Coefficients are scaled by 32767. For pass-through, this is attenuated by
sqrt(0.5) to maintain a consistent perceived volume.
Explicitly include libavutil/channel_layout.h for the channel layout stuff
Use a macro to specify the fourcc formatters
Keep existing presets when loading in a new soundfont

Note that alLoadSoundfontSOFT doesn't currently allow using a soundfont with
existing presets. This is for being able to load and stack multiple soundfonts
for the default soundfont.
Load the default soundfont as a comma-separate list of filename

This allows multiple soundfont files to be "patched" together to create a
single soundfont. For instance a GM soundfont with a separate soundfont for
GS-only additions.
Trace when a soundfont buffer goes unused
Stop trying to read samples if there was an error
Use the UNUSED macro instead of a void cast
Make some functions static