This allows using the "On Cage" feature in edit mode to interact with
original mesh elements via the newly created geometry. The original
indices are only set for new elements that copy attribute values
from original elements directly, so it can also be a helpful way
to visualize attribute propagation.
The change was simplified by refactoring the individual mode slightly
to create separate index maps for the new edges and vertices. That
simplified attribute copying a bit too.
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