let ray tracers respect server side widget visibility

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.