* This direction means Left will cause it to go -x and Right +x so that the directions are correct with respect to "Left" and "Right" when you look "North".
* Always generate in up-direction and rotate to left/right/down
* Generate a "dominant direction" by interrogating the whole curve, not
just the first and last item.
* Improve maths:
- Generate a projection plane spanned by UP and the dominant direction.
- Project the tangent onto this projection plane.
- Generate normal as the cross product between the projected tangent and
the normal to the projection plane.
- Rotate the normal into left/right/down if required.
