This is partially a bug fix, since the original index layer wasn't
handled at all before, and could be left initialized. That was caused
by my recent change to modify a mesh in place rather than create a
new one. Also copy over any orco data, which was another unhandled
layer type.
It's also a new feature though, since it allows using the "On Cage"
feature of edit mode to adjust original mesh elements by selecting
the new ones. This brings the functionality inline with the Edge Split
modifier.
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