Rendering enhancements

[mclarsen]

What is slow and how to fix it:

• Annotations: vtkm scene system adds and renders them one at a time. Batch them
• png encoding: This is inherently serial. Change compositing to handle batches and send final images to different ranks. Encoding can happen happen on all ranks
• external faces (and bvh): this is repeatably done because the vtkm rendering wants it this way. Add re-render when we know things don't change.