Fix for handling negative indices in OBJ loader

Merged Ghost User requested to merge OBJ-loader-negative-indices into master

This fix resolves an issue that occurrs with the current code that uses two passes to parse the OBJ - see my previous merge request:

Sorry for the trouble! :-(

To verify that the negative indices were really not working any more, and to check that this fix resolves the issue, I used the Cornell Box example from the tiny OBJ loader project:

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