梁友栋-Barsky裁剪算法，算法的基本思想：当要裁剪的线段是P0P1， P0P1和窗口边界交于A,B,C,D四点。从A,B和P0三点中找出最靠近的P1的点，从C,D和P1中找出最靠近P0的点。-Liangyou Dong-Barsky clipping algorithm, the algorithm' s basic idea: When you want to cut the line is P0P1, P0P1 and window borders intersect at A,