BUG: Revert default VTK backend to OpenGL
Created by: msmolens
Revert the default VTK backend to OpenGL. Because the OS X factory machine targets OS X 10.6 (CMAKE_OSX_DEPLOYMENT_TARGET:STRING=10.6), the Mac nightly builds don't find the required OpenGL version. The error message seen when running Slicer is:
vtkCocoaRenderWindow (0x125872400): VTK is designed to work with
OpenGL version 3.2 but it appears it has been given a context that
does not support 3.2. VTK will run in a compatibility mode designed
to work with earlier versions of OpenGL but some features may not
work.)