MP: incremental update for terrain layers for add/remove/etc.
Incremental terrain update in MP: almost done, just need a solution for elevation visiblity toggle
Incremental update: make it work with varying LODs and upsampled tiles
Incremental update: add rejection of out-of-date arrivals
Merge branch 'master' into topic-incremental-update
Controls work
Add supplemental nicer-formatted JSON to Config.toJSON
Config: finish 'nicer' JSON output support
Merged incremental update
Merge branch 'master' of github.com:gwaldron/osgearth