Blender Git Loki

Geometry Nodes: Allow reading converted attribute directly from spline

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