This commit adds a field input node that outputs the fraction of the
total length of the spline on each control point. This is useful for
anything that involves varying a value depending on how far along
the spline it is.
It also works when evaluated on the spline domain, where it outputs
the fraction of the total length of all of the splines at the start.
The operation isn't as well defined for NURB splines for the reasons
noted in the code comment. That can be said explicitly in the docs.
Differential Revision: https://developer.blender.org/D12548
Hans Goudey
4d881d9dad