The version reported by the driver in X11 indirect rendering mode is above what
the driver can do (for example it tells it supports 2.1 (Xserver on GeForce
6800) or even 3.2 (Xserver on Quadro FX 3600M)) when it cannot actually support
PBO (pixel buffer objects) which is core in 2.1. It does not advertise PBO as
an extension. The workaround makes vtkOpenGLExtensionManager returns false in
indirect mode for any requested OpenGL version, which let the algorithms to
only tests against advertised extensions.

It has been tested with nVidia driver 195.36.15 (shipped with Ubuntu 10.04).