During VTK refactoring of extents, parallel volume rendering was broken.
Fixing it by bringing back vtkPExtentTranslator (formerly called
vtkPVTrivialExtentTranslator) and making vtkImageVolumeRepresentation
use it to determine image extents across all ranks.

Also added a new test that tests volume rendering in parallel.

Change-Id: I72d06d0bebf14f71f7db6a2a345f668fa63ad16b