[vtkusers] X11 offscreen rendering without Mesa

[Clinton Stimpson]

Just wanted to let some people know that offscreen rendering is possible
without Mesa on X11.  You can use Xvfb.
It isn't a complete replacement for Mesa, but using that instead of Mesa may
be good enough for some people in some cases.  Xvfb even let's you put
entire GUIs in a virtual display, not just the graphics window.  You can
also connect to the virual display at some point during the process to see
its progress.

Clint