This patch refactors the "Hair" data-block, which will soon be renamed
to "Curves". The larger change is switching from an array of `HairCurve`
to find indices in the points array to simply storing an array of offsets.
Using a single integer instead of two halves the amount of memory for that
particular array.
Besides that, there are some other changes in this patch:
- Split the data-structure to a separate `CurveGeometry`
DNA struct so it is usable for grease pencil too.
- Update naming to be more aligned with newer code and the style guide.
- Add direct access to some arrays in RNA
-- Radius is now retrieved as a regular attribute in Cycles.
-- `HairPoint` has been renamed to `CurvePoint`
-- `HairCurve` has been renamed to `CurveSlice`
- Add comments to the struct in DNA.
The next steps are renaming `Hair` -> `Curves`, and adding support
for other curve types: Bezier, Poly, and NURBS.
Ref T95355
Differential Revision: https://developer.blender.org/D13987
f59767ff97