WebGL to WebXR conversion example

This takes a rotating 3D triangle example (based off of this example from MDN) and adds an "enter VR" button that instead lets you inspect the triangle from VR.

You can find the WebGL-only version here, and the converted WebVR version here.

This is a WebXR version of https://github.com/Manishearth/webgl-to-webxr for testing Servo's WebXR implementation.