siting the canvas
        // on the document. For the simplest form of mirroring we want to create
        // the canvas with that option enabled. Note that this may incur a
        // performance penalty, as it may imply that additional copies of the
        // canvas backbuffer need to be made. As a result, we ONLY want to set
        // that if we know the VRDisplay has an external display, which is why
        // we defer WebGL initialization until after we've gotten results back
        // from navigat