第十章的修正“线下”问题

书中第十章的“线下”问题，并没有修正，小球从底部反弹至line的时候，仍然会穿过line到上面去，而不是反弹，将bounce参数调成-1就可以看得很明白了。

更改：如y2的绝对值小于小球半径，则进行反弹，如果y2的值为正则将小球重置到到线下，如为负则重置到线上

if(Math.abs(y2) < ball.height / 2)
{
    // rotate coordinates
    var x2:Number = cos * x1 + sin * y1;
                                        
    // rotate velocity
    var vx1:Number = cos * ball.vx + sin * ball.vy;
                                
    if(y2 > 0) {
        y2 = ball.height / 2;
    }
    else {
        y2 = -ball.height / 2;
    }
                                        
    vy1 *= bounce;
                                        
    // rotate everything back;
    x1 = cos * x2 - sin * y2;
    y1 = cos * y2 + sin * x2;
    ball.vx = cos * vx1 - sin * vy1;
    ball.vy = cos * vy1 + sin * vx1;
    ball.x = line.x + x1;
    ball.y = line.y + y1;
}