Merge pull request #452 from travispereira/master

Update VerticalDatum::transform() to calculate correct lat/lon values
Fix SL/Triton cmake issues; add cloudsMaxAltitude to SL
Merge branch 'master' of github.com:gwaldron/osgearth
Added min-level support for mask layers.
Merge branch 'master' of https://github.com/gwaldron/osgearth