Add a new option that makes the Spline IK solver apply volume
preservation on top of the original scaling, considering the
pre-IK scale of the bone as the goal volume to be preserved.
This basically works similar to the Stretch To constraint, and
allows easily rigging a stretchy chain that uniformly follows
its parent's scaling.
Since the Stretch To behavior is more familiar, the new option
is on by default for newly created Spline IK constraints.
Alexander Gavrilov
37eb109014