how coprocess render the image for distributed cases

[utkarsh.ayachit]

@mdorier

. Our guess is that either (1) the rendering algorithm doesn’t care and uses MPI functions directly, or (2 - more likely) somewhere down the line, something (maybe the Python script itself?) resets the global controller to an MPI one, overriding the one we have installed.

The compositing is done using IceT. IceT doesn’t use VTK-communicator, but directly uses MPI. @Kenneth_Moreland, any suggestions on how IceT’s MPI calls can be forwarded avoid some other mechanism than MPI?