Added automated tests for exporting as OBJ (and then importing again).

Thanks to these tests, I was able to find and solve a lot of issues in particular in features that I never use but that others might. I'm happy.
Added further OBJ export tests.

These revealed, who'd have thought, more bugs and/or areas that could be improved. So now they are.
Test (and fix) OBJ exporting with included textures.
Added tests for export as .mesh

Some of them still fail because the functionality is not built in at the moment.
For models without tangents generate them automatically.

That is not strictly necessary; after all, the only models that will end up lacking tangents are those that don't need them to begin with. But with mesh modding and exporting, life is significantly easier if we can just assume that all meshes can be exported without trouble.
Moved creation of transformed static data to mesh

This is necessary to allow creating static .mesh(.ascii) files. Otherwise, I'd have to have the same code everywhere.