Try to map reverb outputs directly to output channels

Try to map reverb outputs directly to output channels

HRTF and B-Format output still need to use virtual panning directions, but the
reverb works better when it outputs directly to an output channel. Ambient and
directional panning is used to properly mask and attenuate each output channel.

Note that currently the "direct panning" output is quieter than it should be.
Work is underway to attenuate the early reflections and late reverb better.