Challenge: How can we set the rect of a polygon to its visual rect?

[Wilhelm Sanke]

Triangles, pentagons, and hexagons (three-, five-, and six-sided 
polygons) have rects that are different from their visual rects, i.e. 
their opaque areas when the fillcolor is set to true.

As an example, a hexagon with a height of 200 possesses a "visual" 
height of only 172.

A snapshot taken from the graphic has the same dimensions, but the 
resulting image can be cropped to the real visual height.

Cropping however is possible only with images, so how do we proceed to 
set the rect of a hexagon (or triangle and pentagon) to its visual rect?

Any ideas?

Regards,

Wilhelm Sanke
<http://www.sanke.org/MetaMedia>