canvas画圆角矩形的方法

思路：arcTo(x1, y1, x2, y2, r)

参考：https://blog.csdn.net/shi851051279/article/details/80436851

　　http://www.w3school.com.cn/tags/canvas_arcto.asp

　　http://www.365mini.com/diy.php?f=html5-canvas-arcto-demo-2