Issue was caused by BKE_vfont_to_curve() modifying curve->nurbs list which
gave threading issues.
Now added BKE_vfont_to_curve_nubase() which operates on a given nurbs list
base which is local in do_makeDispListCurveTypes().
By the looks of it it wouldn't give speed regression because previously it
also was creating nurbs for every font object sharing the same curve data.
a2bf25e8ff