add support for head-shadow/penumbra filter for positional audio streams

When positional audio sources are behind the listener, we selectively apply low-pass filtering to simulate a HRTF.   Feature may be enabled/disabled using the new audio-mixer settings in domain server. 
Please see comment below.   In general, we'll be introducing quite a bit of floating point math here, these initializations become so much clearer lined up - particularly when there is repeating pattern (each derived filter will have its own similar, but slightly different initialization).  Let me know if this is a really a big issue.