Added DisplaySettings::s/getNvOptimusEnablement() and OSG_NvOptimusEnablement env var control to control the setting of the NvOptimusEnablement variable


git-svn-id: http://svn.openscenegraph.org/osg/OpenSceneGraph/trunk@14614 16af8721-9629-0410-8352-f15c8da7e697