Lithophane Coaster

This is a coaster with a picture that can be viewed by holding it in front of a light source. It is a good idea make a custom coaster for each guest at a party, each coaster with the guest's picture.

The picture is a bw .png photo. .png files are the only recognized by openscad.
The coaster should be printed with a transparent filament in order for light to pass through
the picture, and with 0.2 mm layer height. The program limits the picture to 2 mm in height.
This means that setting infill has no effect on the picture transparency.