Allow NagVis SOURCE to change map beyond simple state changes

I would like some external state to cause a NagVis map to change as directed by a NagVis SOURCE.
I have created a very simple patch to 1.7.10 to do this and hope future releases will have this feature.

I modified GlobalMapCfg::processSources to pass the previously processed sources cache to sourcesChanged (as an additional &$config argument, defaulting to null) which passes it along to my changed_SOURCE function.
My changed_SOURCE inspects a non-null config to see if it should be re-processed or not.

This is much more capable than just comparing a date/time stamp.

I created a singleton class for my SOURCE whose instance is gotten on {process,changed}_SOURCE entry.
Every changed_SOURCE invocation that decides whether process_SOURCE needs to be called increments a counter in the singleton instance and this counter is returned as its result.
This is because a single getObjectStates ajax request may end up calling changed_SOURCE twice (once before the map configuration is read and once after).
Without this logic, the first call would return true, process_SOURCE would fix things and the second would return false - which is returned to the frontend.
We need to return true to the frontend in this case.