objSerializer bug

As shown in code objSerializer, if a model contains two group and the first group contains only v, vn while the second group contains v, vt, vn, then for the f part, the second group will have incorrect vt index. This issue also apply to vn case.

Welcome aboard!

Pinging @Drigax but I think a PG would help because Iā€™m not sure I understand fully the issue.