OverviewRenderer redraws whole overview when box is moved

When the overview window view box is moved, currently the code redraws the whole overview window. This is clearly inefficient and could be avoided by repainting only the region where the box was and now is. (Repainting the box in its own layer might help, but only if we completely separate - e.g. different component - the repainting of this layer from the underlying overview canvas.)