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.

src/osgUtil/CullVisitor.cpp

@@ -1302,6 +1302,8 @@ void CullVisitor::apply(osg::CameraNode& camera)
                 }
                 
                 rtts->setGraphicsContext(context.get());
+                
+                context->realize();
 
             }
         }