This commit adds a new type of volume preservation mode to Spline IK
which makes it possible to set limits on the minimum and maximum
scaling of bone "fatness".
* The old volume preseving mode has been kept but renamed, to avoid
breaking old rigs. "Volume Presevation" uses the new method, while
"Inverse Preservation" is the old one.
* The code and settings for this new xz scale mode are directly lifted
from the improved Stretch To constraint
f0361fcf54