A scene is represented by the <a-scene> element. The scene is the global root
object, and all entities are contained within the scene.
The scene inherits from the Entity class so it inherits all of its
properties, its methods, the ability to attach components, and the behavior to
wait for all of its child nodes (e.g., <a-assets> and <a-entity>) to load
before kicking off the render loop.
<a-scene> handles all of the three.js and WebVR boilerplate for us:
Set up canvas, renderer, render loop
Default camera and lights
Set up webvr-polyfill, VREffect
Add UI to Enter VR that calls WebVR API
Array of components with tick methods that will be run on every frame.
Active three.js camera.
Reference to the canvas element.
Whether or not environment is detected to be mobile.