Blender Git Loki

Partial fixes for issues raised in T44219

* The breakdowner tool will no longer operate directly on properties
of type "enum", as this doesn't make sense most of the time. This
is still not much use though when custom properties (ints) are used
to drive some underlying enum property though (as in blenrig)
* The breakdowner no longer tries to perform any blending if the
start and end values are the same, to avoid float precision issues.