spawn method: do a per active region spawn method

[jtoppins]

Currently all assets are spawned immediately after the theater is generated. To simulate a moving front and load balance the server switch to spawning assets per active region.

Allow the number of active regions to be configurable via a theater configuration option.

[ricmzn]

Would it be a good idea to allow each region to have its own spawn cost, up to a maximum configurable total cost? Smaller regions with less objectives (made so for prioritization sake or even just grouped by design) would not cost as much as more expensive regions, while allowing the map to be populated at a more regular interval. Moving units, if introduced, could also be tied to "high-cost" regions so they only exist while a majority of previous objectives are cleared, as a way of mitigating server performance impact.

Some assets might also want to be spawned regardless of whether a region is deemed as active or not, so that long-range SAMs and CAP can still be a nuisance regardless of campaign progress.

[jtoppins]

Have not thought about the per region spawning in a while.

[jtoppins]

Some interesting progress being made here:
https://github.com/ricmzn/dct/blob/feature-dynamic-spawning/src/dct/systems/RenderManager.lua