In paraview client server mode with server side rendering
widgets can not be hidden with Ray Tracing enabled. The underlying
problem is that vtkSceneGraph deals with a select few vtkProp
child classes (Volumes and Mappers mostly). WidgetRepresentations
in particular were intentionally avoided in the first OSPRay project
and have not been revisited.

Widgets mostly work depite that, because Representations add their
internal Actors to the scene. What doesn't work is that
the containing Representation never gets a chance to control
the children without the accompanying Widget instance, which is not
present on the server side.

This change fixes this widget, but a proper fix will be to do
something like add WidgetRepresentation to SceneGraph.