- Use smooth normals to displace in Above Surface mode.
- Add an option to align an axis to the normal in the constraint.
I've seen people request the alignment feature, and it seems useful.
For the actual aligning I use the damped track logic.
In order to conveniently keep mesh data needed for normal
computation together, a new data structure is introduced.
Reviewers: mont29
Differential Revision: https://developer.blender.org/D3762
e5b18390fa