The solution was to set the defaulcolordepth of the Xserver from 16 bit packed pixel to at least 24 bit true color.
Semms to be an invalid assumption inside tgs or even general open inventor that offscreen renderer takes care of correct visual for xpixmap when doing offscreen rendering. While mesa reutrns indipendently a proper pixmap for truecolor RGB or even RGBA this does not hold for hardwarerendering. (nauturally offscreen hardware offscreen can only support modes the hardware is actually set to
. Open Inventor (TGS VErsion) does not properly handle the non truecolor mode cases. Ergo when tying to acess offscrenn pixmap or pbuffer XServer crashes with sig 11.
Sugest that this will hold for other applications doing offscreen rendering with implicit truefcolor assumption too.