CILViewer via trame requires headless renderwindow

I tested the viewer via trame, to update it with the latest changes in trame. The idea is to see if we can put the viewer in jupyter, for instance. I encountered an issue with trame and remote headless rendering.

https://github.com/Kitware/trame/issues/570

The `renderWindow` needs to be headless:

```python
renderWindow = vtk.vtkRenderWindow()
renderWindow.OffScreenRenderingOn()
```