Using an osg::ref_ptr for 'this' in the constructor is a bad idea

Inside a constructor, the reference count of 'this' is still zero. If you use a local osg::ref_ptr to reference 'this', the reference count will increment to one. However, when the constructor ends, the lifetime of the osg::ref_ptr also ends, and the reference count will become zero again, and 'this' will be destroyed.