Blender Git Loki

Discard non-free axis scaling in Maintain Volume to improve 2.79 compat.

It seems the reason the old version of the constraint overcompensates
as reported in T48079 is to allow the constraint to work with uniform
scaling on all axes. However the way it did that actually _requires_
uniform scaling for the constraint to work correctly, and breaks if
only the free scaling axis is used to avoid redundant channels.

This version attempts to allow both by discarding scaling in the non-
free directions instead of applying the correction on top of it.