splay in spread level center

splay spreads an array of channels across the stereo field.

spread -    0 = mono, 1 = stereo
level  -    0 = silent, 1 = unit gain (equal power level compensated)
center -    -1 = left, 1 = right

> let i = 6
> r <- replicateM i (rand 10 20)
> n <- lfNoise2 KR (MCE r)
> let ci = Constant . fromIntegral
>     x = mouseX KR (-1) 1 Linear 0.1
>     y = mouseY KR 1 0 Linear 0.1
>     o = sinOsc AR (n * 200 + (MCE [1 .. ci i] + 3 * 100)) 0
> audition (out 0 (splay o y 0.2 x))