As far as I can tell, there is no technical reason why the B-Bone
segment thickness scaling can't be separated into two axes. The
only downside is the increase in complexity of the B-Bone settings,
but this is inevitable due to the increase in flexibility.
Updating the file is somewhat complicated though, because F-Curves
and drivers have to be duplicated and updated to the new names.
Reviewers: campbellbarton
Subscribers: icappiello, jpbouza
Differential Revision: https://developer.blender.org/D4716
624e93bbef