A point% is used for certain drawing commands. It
 encapsulates two real numbers.
| (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.