在一个已有的PictureBox1上添加一个PictureBox2,PictureBox2覆盖PictureBox1上面,PictureBox2的backcolor设置为Transparent,直接穿透PictureBox1,显示窗体背景颜色.。怎么让PictureBox2显示PictureBox1的颜色,不让他穿透PictureBox1呢?