The removed code was necessary due to a logical bug in the use of CD_ORIGINDEX
layer entries. Previously the index values were used to refer to the edges/polys
of the derived mesh, assuming that the derived mesh edges/polys consisted of those
from the original mesh plus newly added edges/polys. This assumption was not
correct, although it worked well for several modifiers such as Mirror and Solidify.
Now the index values are specifically used to reference the edge/polys of the
original mesh.
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