Change a way of drawing of bounding sphere around openvrml objects.

1. vrml97 and other nodes do not call viewer::draw_bounding_sphere()
directly. Instead the method is called from child_node::render_chield()
2. The rendering_context now has a pointer to a node that needs bounding sphere
to be painted around it.
3. The browser::render() method (and nodes that work with the rendering_context)
does not create the context, instead it receives the context from outside.