ENH: add SetInputData() in vtkVolumeMapper.

To remove the GetProducerPort() in VolumeRendering module in Slicer, a SetIntpuData(int port, vtkImageData *) is needed in vtkVolumeMapper.
http://www.vtk.org/Wiki/VTK/VTK_6_Migration/Removal_of_GetProducerPort.