Blender Git Loki

Armature: implement lengthwise scaling of B-Bone segments.

Implement actual behavior for the B-Bone Y Scale channels added
to DNA and UI in the previous commit in addition to the existing
X and Z Scale inputs.

The two length scale inputs control the ratio between the lengths
of the start and end segments of the bone: although for convenience
two inputs are provided, the whole chain is still uniformly scaled
to fit the curve.

Differential Revision: https://developer.blender.org/D9870