Create `Curves` directly, instead of using the conversion from
`CurveEval`. This means that the `tilt` and `radius` attributes
don't need to be allocated. The old behavior is kept by using the
right defaults in the conversion to `CurveEval` later on.
The Bezier segment primitive isn't ported yet, because functions
to provide easy access to built-in attributes used for Bezier curves
haven't been added yet.
Differential Revision: https://developer.blender.org/D14212
444d57d440