anchor.diff:        List<ShaderObject> fxShader,
         near: 0.1,
         far: 1000.0,
  +      bool preserveDrawingBuffer: false,
         bool addDefaultRenderingPhase: true,
         bool useElementIndexUint: false}) {
       if (canvasOrID is HTML.CanvasElement) {
  
positive.code:     _camera = new Camera();
  
      if (addDefaultRenderingPhase) {
        _renderPhases.add(new RenderingPhase(gl, null, true));
      }
      setUpEventCapture(_canvas);
    }
  
negative.code:     _canvas.width = perspar.width;
      _canvas.height = perspar.height;
      //_aspect = _canvas.clientWidth / _canvas.clientHeight;
      gl = _canvas.getContext("experimental-webgl", {"preserveDrawingBuffer": preserveDrawingBuffer});
      if (gl == null) {
        throw new Exception(
            'calling canvas.getContext("experimental-webgl") failed, make sure you run on a computer that supports WebGL, test here: http://get.webgl.org/');
  