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. 
another subtle coding standard issue. The standard explicitly says not do these sorts of spacing to create alignment for blocks of code like this. The logic behind this coding standard rule is that when someone has to come back to change or add code, if they change the length of things on one side of the alignment they end up having to spend time as a "text formatter" instead of a developer. So we've explicitly said not to do this.