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.
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