Previously Spline IK provided only two choices: either scale the
length of the bone chain to fit the length of the curve, or don't
scale the bone in the Y dimension at all (ignoring effects of
actually fitting to the curve due to curvature and curve object
scale).
This patch adds a new option to use the pre-IK Y scale of the
bones to adjust their length when fitted to the curve, allowing
individual posing control over the length of the segments.
Reviewers: brecht
Differential Revision: https://developer.blender.org/D4687
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