Waveshaper curve interpolation algorithm incorrect

[rtoy]

Describe the issue

See https://webaudio.github.io/web-audio-api/#dom-waveshapernode-curve which says, in part:

y = (1-f)*c[k] + f*c[k]

If you expand this out you get 1.

I think the formula should be

y = (1-f)*c[k] + f*c[k+1]

When v is just slightly larger than k, f = v-k which is small, so we want most of the contribution to be from c[k], and very little from c[k+1].