Blender Git Loki

Bugfix #3772: Allows "Center Cursor" for 3D curves to modify the curve
center's Z location. An eons-old comment in the code said:

/* Curves need to be 2d, never offset in
* Z. Is a somewhat arbitrary restriction,
* would probably be nice to remove. */

I couldn't find any other reason for the restriction other than 2D curves
limit the point's Z component to 0, so added a check to only forve the
center Z to 0 when the curve is 2D. If there are other reasons for the
restriction, then this commit may need to be rolled back.