as osg::GraphicsOperation. Unpdated parts of OSG depending upon these.
Added a virtaul bool valid() method to osg::GraphicsContext to allow apps to
test whether a valid graphis context has been created or not.
added frame stamp updating and update traversal to osgViewer::Scene/Viewer.
Updated osgcamera example to use new Viewer API calls instead of using local
rendering calls.
Added osgcamera example that uses osg::GraphicsContext to create the required
window for rendering too, will eventually use osg::CameraNode to replace usage
of osgUtil::SceneView.
Added options into osgprerender for controlling how to do the pre rendering i.e.
--fbo, --pbuffer, --fb --window, and also added the option for controlling the
window size with --width and --height.
