Added realize() and isRealized() methods to osg::GraphicsContext.

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.

Make/makedirdefs

@@ -176,6 +176,7 @@ EXAMPLE_DIRS = \
 	osgblendequation\
 	osgcallback\
 	osgcatch\
+	osgcamera\
 	osgcameragroup\
 	osgclip\
 	osgcluster\