Automatically setup the render loop

Previously, users had to manually call ```Scene.render()``` each frame. Most users don't need to maintain their own render loop, so now it will be automatically setup. Users can still provide ```preRender``` and ```postRender``` function hooks, for example for tracking performance or other stats. Optionally, users can pass the ```disableRenderLoop``` option to the ```Scene``` constructor if they want to manually manage the loop themselves.