Reduce scope of sunlight shadowcasting.

[kevingranade]

Is your feature request related to a problem? Please describe.
During active play, recalculating sunlight levels in outdoor areas consumes a significant amount of processing time.

Describe the solution you'd like
On map shift or on initial sunlight propogation, we can track which horizontal map slices are populated with opaque tiles, and start sunlight propagation at that level instead of starting at the top of the skybox.