1) Fixed ray casting for rendering a large image
2) Removed RGBTexture input from the volume mapper
3) Modified vtkVolumeProperty to support properties per "field"
   with a maximum of 4 fields.
4) Remove all RGBTexture parameters from the property
5) Small fix to interactor style joystick camera for endpan and endzoom

Most of this work is in preparation for multiple component support
for volume rendering in VTK.