[osg-submissions] Check GLX Version in pbuffer init

[Mathias Fröhlich]

Hi Robert,

Attached is a change to the PBuffer initialsation code that checks for the GLX 
version before issuing commands that require GLX-1.3.

This prevents a crash with open source drivers on linux.

The change is based on rev 7644.

Please apply

    Thanks

         Mathias
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