New snapshot (20100807) - Spatial sensitivity for server-side caching of tiles for user layers

This snapshot is a follow-up to yesterday's recent changes.  It adds spatial sensitivity to the server-side caching of tiles for user layers (e.g., only tiles that are affected by more recent changes in user drawings will be re-rendered).  It also includes a fix that handles the client-side reaction to changes in user drawings - the timestamp parameter for the user layer is updated any time user drawing changes are detected.  This means that all map-tiles for the users layer will ultimately be re-downloaded on a zoom-pan.  The previous behaviour only refreshed tiles that were affected if they were currently within the map display, but without updating the timestamp for the layer, any cached tiles outside of the map view or at different zoom levels would not be updated.  Overall, this appears to be the best solution without implementing much more complex mechanism on the client side for keeping track of which tiles need to be refreshed.
 
Updates are posted on the downloads page.