Merge branch 'OBJ-loader-negative-indices' into 'master'

Fix for handling negative indices in OBJ loader

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

See merge request !93
7 jobs for 33-triangulation-via-ear-clipping in 21 minutes and 9 seconds
Status Job ID Name Coverage
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passed #11804
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clang-c++11

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gcc-c++11

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passed #11805
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gcc-c++98

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Apple
macos-c++11

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macos-c++98

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