ogt_vox: group references broken by commit: flatten animations ...

[dougbinks]

Commit 137ff3d breaks the handling of group references.

Steps to reproduce:

1. Create a scene in MV with several models
2. Group the models
3. Create a reference (SHIFT+move or use icon) and move the new reference somewhere.
4. Open with ogt_vox before/after this commit. Scene is incorrect after this commit.

Example vox file: https://drive.google.com/file/d/1qpp7WUnk7_w02dCvjI0V3VTLA332dkWq/view?usp=sharing

EDIT: note that for me this fix isn't urgent as I'm able to use the previous commit plus cherry picked resize perf fix as I don't support animations for now.

[jpaver]

Thanks for the details, PR that fixes this is here: #36.
I've also opened this issue (#37) to add formal test cases to prevent this sort of regression in the future. Thanks again!