A point% is used for certain drawing commands. It
encapsulates two real numbers.
constructor
(make-object point%) → (is-a?/c point%)
(make-object point% x y) → (is-a?/c point%) x : real? y : real?
Creates a point. If x and y are not supplied, they
are set to 0.
Gets the point x-value.
Gets the point y-value.
Sets the point x-value.
Sets the point y-value.