Fast pitch coefficient approximation for filter12

[olofson]

There's powf(), sin() and division going on in filter12 when changing the cutoff frequency. That's not very fast - and if we're going to run this on non-FPU hardware, it's really bad news!

We could probably come up with a polynomial approximation for the whole expression, that's faster (at least without a proper FPU) and still accurate enough for musical pitch tuned filters.