function convertMatrix(source, target) {
    for (var n = 0; n < 16; n++) {
      target.elements[n] = source[n];
    }
  }
Renders the scene to both eyes.
@param {!THREE.Scene} scene Three.js scene.
@param {!THREE.Camera} camera User camera. This is treated as the neck base.
@param {vr.VRState} vrstate VR state, if active.