BUG: Revert default VTK backend to OpenGL

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.)

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25406 3bd1e089-480b-0410-8dfb-8563597acbee