canvas 曲线优化, 3阶贝塞尔曲线

https://www.jianshu.com/p/8f82db9556d2

https://www.cnblogs.com/zl03jsj/p/8048102.html

https://www.iteye.com/blog/liyiwen-705489

https://segmentfault.com/a/1190000016672567

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https://blog.csdn.net/u014070279/article/details/47152335