[plt-scheme] stars in image teachpack

The star procedure currently checks that the given number of points are 
in the real[!?] range (3,10000).  I wonder if this could be relaxed to 
(2,10000).

A three pointed star makes sense to me and removed the check doesn't 
cause any code to crash when constructing these stars.

Moreover, star then becomes very useful for generating regular 
(even-sided) polygons.  In particular, I want to be able to do:

   (define (hexagon size mode color)
     (star 3 size size mode color))

The real restriction is a bug; making a star with 4.2 points causes 
DrScheme to loop.

David