X-major lines broken in poly.5c

Hi Keith,

Here's a trapezoid that tests X-major lines of both slopes. Your code
gets one right, one wrong:

Trap::trap t3 = Trap::new (1.5, 15.5,
                           Line::new (Point::new (10, 1),
                                      Point::new (60, 31)),
                           Line::new (Point::new (80, 1),
                                      Point::new (30, 31)));

Currently my code gets both of these wrong -- but in the same way at
least. ;-)

-Carl