Separate Overlays into individual graphics contexts, for more efficient updates

[cmhulbert]

Currently, when the screen is updated, all of the overlays are also updated, since they share a graphics context. However, this is not always necessary. It should be possible to separate the overlays, and only update the individual overlays when they change. Potentially, this could also implement overlays that are localized to a particular region of the screen, to enable partial screen overlay updates.