The previous implementation defined Compare_xy_2 as
simply Compare_xyz_3. This does define an order
over the projected points, but it has a strong downside:
it ignores the direction of the normal, so its order
can be the opposite of the lexicographical order
would give. This new order is in conflict with
the existing Compare_x_2 and Compare_y_2, which
take the normal's direction into account.
(Also with the Less_x_2, Less_y_2, and Less_xy_2
family, which also exist for the projection traits).
