BUG #14794: Reenable parallel volume rendering for images.

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
Merge topic '14794_fix_parallel_image_volume_rendering' into next

1a51d03f BUG #14794: Reenable parallel volume rendering for images.