The loose edge section of the index buffer was built incorrectly,
essentially assuming that all the hidden edges came at the end of the
loose edge indices. To correct this, we need to track the visibility of
each loose edge index rather than the visibility of each edge itself.
This is driven by the need to combine the loose and non-loose indices
into one IBO with the loose edges in a contiguous section, and the
desire to avoid restart "invisible" indices in the buffer to upload
less data to the GPU.
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