LOD merge with tangents fails

[erikdahlstrom]

LOD merge causes the tangents from the highest LOD to be used where it really should use the tangents from the appropriate LOD instead.

I think all that's needed is a line in GLTFLODUtils.cpp::AddGLTFNodeLOD:
...
AddIndexOffset(primitive.tangentsAccessorId, accessorOffset);
...