FixedPoint volume rendering does not work

[lassoan (Andras Lasso)]

@Frank_Gao

I think we have to register the object factory for OpenGL2 module under volume rendering. Not sure why this is not registered by default when the class is used.

The class you need to instantiate for single-volume GPU volume rendering is vtkGPUVolumeRayCastMapper it is up to VTK to decide which exact implementation class to instantiate, depending on the current rendering backend.