Often it would be beneficial to avoid the virtual array implementation in `geometry_component_curve.cc` that flattens an attribute for every spline and instead read an attribute separately for every input spline. This commit implements functions to do that. The downside is some code duplication-- we now have two places handling this conversion. However, we can head in this general direction for the attribute API anyway and support accessing attributes in smaller contiguous chunks where necessary. No functional changes in this commit. Differential Revision: https://developer.blender.org/D11456