Clear up length VCB input

[thomthom]

Currently it says "Distance", and in the viewport you see a number for the radius. But when entering a length instead of an angle it represent the sagitta.

Need to make this clearer.

[thomthom]

Compute angle from sagitta and arc length: https://gist.github.com/thomthom/dcb8e1e9d9e301272a3733a826e19284

[thomthom]

Using the sagitta becomes a challenge. Instead the distance is a length relative to the reference segment length. The ratio between these becomes the ratio for a 360 degree bend.