Currently, only a global instance of RenderingManager exists, which takes care of all created instances of BaseRenderer / vtkRenderWindow.

In various situations it would be helpful to have multiple instances of RenderingManager, each being responsible for a number of render windows. For example, the four standard windows would be assigned to one RenderingManager instance, while the render windows in a radiological light-box would be assigned to another instance. See also T3341.

Need to think about how RenderWindow instances register themselves to a specific RenderingManager (e.g. there could be a default RM instance which is used if no other RM instance is specified at render window construction).