ENH: VTK6 migration, use system zlib.
ENH: VTK6 migration, replacement of AddInput().

http://www.vtk.org/Wiki/VTK/VTK_6_Migration/Replacement_of_SetInput.
ENH: address vtk6 library linking issues.
ENH: VTK6 migration, additional replacement of SetInput().

http://www.vtk.org/Wiki/VTK/VTK_6_Migration/Replacement_of_SetInput