Previously, things like materials, symmetry, and selection options
stored on Curves weren't copied to the result in nodes like the
subdivide and resample nodes. Now they are, which fixes some
unexpected behavior and allows visualization of the sculpt mode
selection.

In the realize instances and join nodes the behavior is the same as
for meshes, the parameters are taken from the first (top) input.

I also refactored some functions to return a CurvesGeometry by-value,
which makes it the responsibility of the node to copy the parameters.
That should make the algorithms more reusable in other situations.