pv_BrickWall buffer wipe

Clears bins above or below a cutoff point.  `wipe' = a unit signal,
from -1 to 0 the UGen acts as a low-pass filter, from 0 to 1 it acts
as a high pass filter.

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

> do { n <- whiteNoise AR
>    ; let x = mouseX KR (-1) 1 Linear 0.1
>      in audition (out 0 (ifft' (pv_BrickWall (fft' 10 (n * 0.2)) x))) }