Implement Gordon-Smith 2-pole oscillator

+ ensure that new audio path is completely out of the way when it is not enabled (e.g., extra buffer copies)
Ok @AndrewMeadows  - I don't feel strongly about this. But the coding standard is definitely not clear on this. I'm not sure what other cases we have in the code for this nuance. I'm totally OK with "pointers to types use the same casting as their types"... that sounds like a good rule to me. But let's clarify and document in the standard.