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. 
yes - cool...agree.  Leaving the audiofilter.h alignment in for now because this pattern will be repeated for each derived filter type, e.g., the coefficient calculations.   If this is really bad form; however, I'll certainly change it.