Rework PANOSE generation

Rework PANOSE generation

The code predated the introduction of bold variants, so it was
significantly incorrect.  Also it appears that some software uses the
PANOSE classification to distinguish monospaced fonts.  After careful
study of the specification, I think all the variants of Bedstead can
be classified as text fonts, so I've removed the code that described
some variants as decorative.  Then I did some calculations and a lot
of playing around with GeoGebra to work out which parts of the design
space give which values of Stroke Variation, Letterform, and Midline.