Renamed the osg::VisualsSettings to osg::DisplaySettings, and
osgUtil::VisualsRequirementsVisitor to osgUtil::DisplayRequirementsVisitor.
Added support for OSG_SCREEN_HEIGHT into osg::DisplaySettings, and added
a DisplaySettings* to the constructors of osg::SceneView and osg::Camera.
these global attributes are created by cloning any attributes which are
applied during rendering, the clone in a shallow copy, which will set up
default valus for that attribute. This should prevent attribute bleed
from one stateset to the next when the global StateSet doesn't contain
an attribute used within the scene graph.