pv_RandComb buffer wipe trig

Randomly clear bins.

buffer = fft buffer.  wipe = clear bins from input in a random
order (0, 1).  trig = select new random ordering.

> withSC3 (\fd -> async fd (b_alloc 10 2048 1))

> let { x = mouseX KR 0.6 0.95 Linear 0.1
>     ; t = impulse KR 0.4 0 }
> in do { n <- whiteNoise AR
>       ; c <- pv_RandComb (fft' 10 (n * 0.5)) x t
>       ; audition (out 0 (pan2 (ifft' c) 0 1)) }