实现连续的贝塞尔曲线原理：
http://blog.iderzheng.com/continuous-and-smooth-bezier-curve/
前一条曲线的最后一个控制点和终点的位置已经决定了下一条曲线起点和第一个控制点的位置
画连续的贝塞尔曲线方法：
http://www.jayway.com/2012/08/29/creating-custom-android-views-part-3-animating-your-custom-views-smoothly/
A good way of picking them is to place the first control point on a line that goes through the starting point (point i) and has the same slope as the line between point i-1 and point i+1. The second control point should be on the line that goes through the end point (point i+1) and has the same slope as the line between point i and point i+2. This will produce a path that is continuous and whose derivative is also continuous. In other words, it will look smooth.