By calling getSnapshot on a CreditSystem, clients can get the current set of active credits, as well as the set of credits that were removed since the last snapshot. However, this only works across all TilesetViewGroups. There's currently no easy way to get this information per view group. This is unfortunate, because some clients would like to, for example, show separate credits in each viewport.

To support such clients, let's add a getCreditsSnapshot method to TilesetViewGroup. This method can walk the _previousFrameCredits and _currentFrameCredits collections to determine which credits are active, and which were removed in the current frame.