Make sure all gain steps are applied with the SSE and Neon mixers
Clamp the current and target gain lower bound to epsilon

Should give a bit more wiggle room for the gain stepping to get lower than the
silence threshold.
Better pack HRTF mixing properties
Always use the current gains when mixing

The current gain gets explicitly set to the target when the stepping is
finished to ensure the target is still used. This way, however, will allow for
asynchronously 'canceling' a fade by setting the counter to 0.
Remove unnecessary ifdefs

mixer_sse.c and mixer_neon.c are only compiled when the relavent headers are
found anyway.
Don't use the next power of two when duplicating config values

Speed isn't terribly important here, and reallocs should be extremely rare.
Use ALuint for the amount to skip