Blender Git Loki

Fix T51971: IK non-uniform scale regression.

This is a different fix for the issue from D2088, preserving backwards compatibility
for IK stretching. The main problem with this patch is that this new behavior has
been there for a year, so it may break rigs created since then which rely on the new
IK stretch behavior.

Test file for various cases:
https://developer.blender.org/diffusion/BL/browse/trunk/lib/tests/animation/IK.blend

Reviewers: campbellbarton

Subscribers: maverick, pkrime

Differential Revision: https://developer.blender.org/D2743