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.
