Blender Git Loki

Counteract numerical errors in distance constraint evaluation (drift).

With large steps in hair editing strokes the solution to distance-constrained
hair displacement becomes inaccurate, leading to a gradual lengthening of strands.
To counteract this the length constraints can enforce a velocity relative to the
constant target length of each segment. This results in an overall constant
length, even when the constraint solution is inaccurate for nonlinear constraints.

Note that the result would become much better with backward Euler integration,
currently there is a noticable 1-step lag between strokes and the distance fix.