surface is inconsistent with Bézier spline

[ijlyttle]

Although the control points are constrained to a surface, the Bézier spline is not.

This results in an inconsistency (note the max chroma):

[ijlyttle]

I don't know if this is a real problem or not.

If we wanted to constrain the spline to the surface, we could - but we would lose the Euclidean-distance rescaling that we get "free" as a part of the spline.

We could write our own rescaler, based on farver::compare_color(), but that might be a bit much.

[ijlyttle]

solved in #51