fix issues when using mixed-mode pos-invariant vp and ff tnl (bug #9856)

fix issues when using mixed-mode pos-invariant vp and ff tnl (bug #9856)

ARB_vp requires vertex transformation to be invariant to fixed function tnl
if the position_invariant option is used. So the same function needs to be
used, otherwise z-fighting artifacts may happen with applications which rely
on the results being really the same due to precision issues when dealing with
floating point values (may not be a problem when using a non-optimizing
compiler strictly following IEEE rules).