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. 
I won't hold up this merge for this issue. But you should chat with the other developers about this and about potentially changing the coding standard and/or adding explicit exceptions for this sort of thing. As you said, I didn't make the rules, I just follow them.