This fix resolves an issue that occurrs with the current code that uses two passes to parse the OBJ - see my previous merge request: https://graphics.rwth-aachen.de:9000/OpenMesh/OpenMesh/merge_requests/84
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: https://github.com/syoyo/tinyobjloader/blob/master/models/cornell_box.obj