Do we need in any case a concept for updating the render windows other than by using RequestUpdateAll / ForceImmediateUpdateAll?
It may be possible to update only those render windows that currently show the same image as the topmost visible image.
It may also be possible to only update those render windows that are in any way semantically related, e.g. images of the same control point etc.
This is connected to the question, if we want to handle multiple rendering managers (e.g. one for each multi widget editor).