Enable ATI graphic cards on Mac OS X
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.