trapezoids

[David Feuer]

Lilypond draws beams as polygons.  The polygon code (in lookup.cc) is
hairy, and probably slow.  It'd be much easier and faster to make
beams trapezoids.  More generally, from reading the comments on the
polygon code, I get the feeling it might be best to get rid of it
altogether.  I'm looking through the old polygon thread now, and I'll
see if I can come up with a good solution.