Make sure that the calculated deco ends at 0

Without this the cairo_close_path call could do silly looking things
(intersecting polygons...).

Reported-by: "Robert C. Helling" <helling@atdotde.de>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Clean up DEBUG code

It's still a mess (and the symbols aren't used consistently), but it's a
tiny bit more logical...

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>