title: Paraview python use offscreen image rendering
body: <p>I'm trying to generate images using Paraview in a non-interactive PBS job on a remote linux machine. From the command-line, if I have a file called <code>cone.py</code> with the following contents</p>
  
  <pre><code>from paraview.simple import *
  Cone()
  Show()
  SaveScreenshot('cone.png')
  </code></pre>
  
  <p>and I type <code>pvpython cone.py</code> in the command-line, then a window pops up showing me the image of the cone, and the image is saved. I don't want the window to pop up. It does even if I use <code>pvbatch cone.py</code> or <code>pvbatch cone.py --use-offscreen-rendering</code>. If I try to run this code from within a PBS job, the image isn't generated (probably because it can't generate the window) and the following error message is generated:</p>
  
  <pre><code>ERROR: In /home/kitware/Dashboards/buildbot/paraview-debian4dash-linux-shared-release_qt4_superbuild/source-paraview/VTK/Rendering/OpenGL/vtkXOpenGLRenderWindow.cxx, line 542
  vtkXOpenGLRenderWindow (0x139559c0): bad X server connection. DISPLAY=/var/spool/PBS/mom_priv/jobs/1443323.rrlogin.internal.SC: line 8: 21926 Aborted                 pvbatch cone.py
  </code></pre>
  
  <p><code>/home/kitware</code> isn't a valid directory on my machine. Any help would be appreciated.</p>