WIP: Small improvement in drawing endpoints

Make it clear with variable names that the polylines_to_draw list
is only for drawing, not for calculations.
Get rid of decimal.Decimal objects entirely

No more confusion over whether we're using Decimal objects or floats;
now that we're no longer using sets to eliminate triangulation lines
that are also sides (but comparing vectors instead within an error
margin of epsilon), we no longer need Decimal objects. Since they don't
interoperate well with floats, this is a big win for code maintainability.
WIP: Drawing midlines in same direction

Some progress: several points in a row in right direction. Still
need to handle "skip to next endpoint" situations right (right now
it draws the line, which is wrong) and also figure out why it was
skipping "early". But it's a good start.