Pros are avoiding copies of spline attributes, avoiding copies when
only a few splines are selected. Cons are potentially copies when
the input curve has multiple users. The code is also simpler.

I expect using the selection input is pretty rare, but when you
actually only want to resample a few splines, it feels a bit silly
to duplicate the entire curve.

Combined with some CoW behavior when copying CurveEval,
this probably gets us the best of both worlds.