[4.0] GIF is not animated in image shape for g-canvas

[dengfuping]

• [x] I have searched the issues of this repository and believe that this is not a duplicate.

Reproduction link

Steps to reproduce

• See the demo of g-canvas

What is expected?

• GIF should be animated for g-canvas

What is actually happening?

• GIF is not animated for g-canvas.

Environment	Info
g-canvas	0.1.0-beta.7
System	Mac OS
Browser	Google Chrome 77.0.3865.90

[xiaoiver]

Canvas 绘制 GIF： https://stackoverflow.com/questions/48234696/how-to-put-a-gif-with-canvas

亦可参考 konva.js 的实现： https://konvajs.org/docs/sandbox/GIF_On_Canvas.html