86df1d25 OBJReader: fix assigment of material id for each cell

Acked-by: Kitware Robot <kwrobot@kitware.com>
Acked-by: buildbot <buildbot@kitware.com>
Reviewed-by: Mathieu Westphal (Kitware) <mathieu.westphal@kitware.com>
Reviewed-by: Nicolas Vuaille <nicolas.vuaille@kitware.com>
Merge-request: !11916