Expand the scenegraph code, and create usage demo.

[mdsitton]

Currently the scenegraph is very early stages, it hasn't even really been tested for rendering very thoroughly or at all.

Its also missing some stuff that I'm convinced could be used to better manage opengl calls, which should improve performance.

[AlexMarinescu]

Any way we can add custom "render" function? Right now you would call scengraph.render() for all. However, there are some situations where I needed to have a custom render call so I wouldn't be forced to overwrite the default one.