slide_shape 6 (squared) and 7 (cubed) don't work with negative values

[jweather]

n = play 70, pan: -1, pan_slide: 1, pan_slide_shape: 6
control n, pan: 1

The sound never pans to the right. Changing the shape to any other value works correctly. Changing the starting pan value to 0 works correctly.

[samaaron]

There's definitely something fishy going on here and it looks like it's related to Overtone's implementation of varlag (which attempts to mirror that in SuperCollider lang).

Needs further investigation...