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. 
btw - there are multiple places in the PR where you've got this construct. I only marked one of them. Same with the horizontal lines thing. I didn't want to flood you with the same message over and over again.