font and platform dependency removed. Refs: #32.
As proposed originally by Ivan.
Implementation partially inspired by Ivan's triangle version.
Uses device coordinates for drawing in order
to avoid rounding problems and thereby good rendering results.
Uses rectangles and 45 degree angles only and tries to avhieve
a high degree of symmetry and scaling for the fold shapes.

Signed-off-by: Josef Templ <josef.templ@gmail.com>