如何在上面的fabric.js 1.7中使用'clipto‘函数

var canvas = new fabric.Canvas('canvas');

var rect = new fabric.Rect({
  left: 50,
  top: 50,
  width: 200,
  height: 200,
  fill: 'red'
});

var circle = new fabric.Circle({
  left: 100,
  top: 100,
  radius: 100,
  fill: 'blue'
});

rect.set({
  clipTo: function(ctx) {
    circle.render(ctx);
  }
});

canvas.add(rect);

var canvas = new fabric.Canvas('canvas');

var rect = new fabric.Rect({
  left: 50,
  top: 50,
  width: 200,
  height: 200,
  fill: 'red'
});

var circle = new fabric.Circle({
  left: 100,
  top: 100,
  radius: 100,
  fill: 'blue'
});

rect.set({
  clipTo: function(ctx) {
    circle.render(ctx);
  }
});

canvas.add(rect);