font: rewrite and parametrize natural code

font: rewrite and parametrize natural code

The logic remains virtually the same. List of changes:
* more descriptive variable names,
* removed surplus points (1', 3', 11', 21'),
* stem brushing is controlled in a simpler way,
* beam slant does no longer depend on stem thickness,
* all dimensions are explicitely and straightforwardly
  derived from global parameters.  This makes it trivial
  to get a differently-looking natural by just changing
  clearly defined values.

At size 20, the natural remains identical.  At signifincantly
different sizes there are a few microscopic changes, but they
are too small to be noticeable:

* stem end thickness is defined in terms of stem thickness,
  so regardless of design size the ratio between them is constant.
* beam slant is slightly more consistent.  But you wouldn't notice
  the change if i haven't told you ;-)