CanvasContext.draw报错？

2D方式后，获得的不是CanvasContext而是RenderingContext

参考：

canvas | 微信开放文档

https://developers.weixin.qq.com/miniprogram/dev/component/canvas.html#Canvas-2D-%E7%A4%BA%E4%BE%8B%E4%BB%A3%E7%A0%81

RenderingContext | 微信开放文档

https://developers.weixin.qq.com/miniprogram/dev/api/canvas/Canvas.getContext.html

RenderingContext是没有draw方法的