Nested shells in MeshToVolume conversion

Hello,

I am having some trouble converting a mesh with nested shells in the MeshToVolume conversion function. I am getting a grid that shows the entire volume as filled rather than just the skin. Please see:
[nestedShell.zip](https://github.com/dreamworksanimation/openvdb/files/644767/nestedShell.zip)

for a sample file.

It does say that the normals are not taken into account during the conversion, so I'm not sure than how else to handle this -- maybe do a csgSubtract of the inner shell from the outer shell? Thanks!

Brad

Start with the MeshToVolume conversion function and reproduce the reported behavior using the attached nestedShell.zip sample. Determine why nested shells produce a fully filled volume rather than only the skin, and verify that the resulting conversion preserves the expected nested-shell geometry.

Written by the indexing model from the issue text.