TMap< FViewportClient *, int32 > InvalidationMap
Tools need to be able to Invalidate the view, in case it is not Realtime. Currently we do this very aggressively, and also force Realtime to be on, but in general we should be able to rely on Invalidation. However there are multiple Views and we do not want to Invalidate immediately, so we store a timestamp for each ViewportClient, and invalidate it when we see it if it's timestamp is out-of-date. (In theory this map will continually grow as new Viewports are created...)