Fixes to OpenCL Volume Rendering

Not working quite yet, but the slicer volume is now loaded as a 3D
image for rapid linear interpolation on the GPU and the rendered image
is brought back as a buffer that can be interpreted as a vtkImageData.

Next steps before basic functionality works:
* work out exact numpy array type required for loading the volume
* work out array type for transfer function
* map camera parameters to invViewMatrix
* apply transforms to volume

Medium term goals:
* perform timing tests and optimize quality/performance
* render multiple volumes
* apply nonlinear transforms on the fly
* generate lookup table using lable map

Longer term wish list:
* integrate with image buffer and z buffer from vtk rendering
* add lighting
* add masking