Enable GPU Raycast volume mapper for recent NVIDIA drivers on Mac OS X

GPU Raycast mapper was crashing for GL_VERSION "2.1 NVIDIA-1.6.XX".
However it works fine for tested version "2.1 NVIDIA-8.0.61"
GPU raycast volume rendering now supports textures larger than 1024

When dealing with large opacity or color transfer functions, shrinking the
size of the function might add some artifacts.
This can be an issue with labelmaps where each entry has a color
very different from its neighbors.
BUG: fix null pointer crash in view to world with no active camera

When Slicer layouts change, the slice node can be set and trigger an
update of widget positions before the renderer is fully set up. This
change avoids a null pointer crash caused by the renderer not checking
before dereferencing the ActiveCamera pointer. Match the error message
printed by the WorldToView method at line 1303, return 0,0,0.

Slicer Issue #3761
Configure VTKConfigVersion consistent with VTK's compatibility guarantees

Since the minimum CMake version required by VTK is 2.8.8, this commit
uses the module CMakePackageConfigHelpers available in that version of
CMake.