The original mistake I made in b9febb54a492ac6c938 was thinking
that the input curve object data to BKE_displist_make_curveTypes
was already copied from the original. I think I misread some of its
ID flags. This commit places the result of curves evaluation in a
duplicated curve instead, and copies the edit mode pointers
necessary for drawing overlays. Curve needs to know not to
free those pointers.

I still don't have a full understanding of why some of the tactics I've
used work and others don't. I've probably tried around 8 different
solutions at this point, and this is the best I came up with.

The dependency graph seems to have some handling of edit mode
pointers that make the edit mode overlays work if the evaluated
result is only an empty curve created by the evaluated geometry set.
This doesn't work with the current method and I need to set the
edit mode pointers at the end of evaluation explicitly.

We're constrained by the confusing duality of the old curves system
combined with the new design using the evaluated geometry set.
Older areas of Blender expect the evaluated Curve to be a copy
of the original, even if it was replaced by some arbitrary evaluated mesh.