Added read callback to ElevationQuery to allow paged models to fully load.
Added check for modifying shared VirtualPrograms when environment var is set.
Added ability to log an individual VirtualPrograms' shaders to a file or std::out.
Merge branch 'master' of https://github.com/gwaldron/osgearth
Merge branch 'master' of https://github.com/gwaldron/osgearth
Merge branch 'master' of https://github.com/gwaldron/osgearth