https://blog.csdn.net/chelen_jak/article/details/80518973
计算两条相交线的角度和弧度
Point S1 ;
Point E1;
Point S2 ;
Point E2 ;
var a = Math.Atan2(S1.Y - E1.Y, S1.X - E1.X);
var b = Math.Atan2(S2.Y - E2.Y, S2.X - E2.X);
var c = 180 * (b - a) / Math.PI);