Structure:
The structure is pretty complex.
SEGMENTS:
- The dictionary has points as keys. Each point links to every segment
having it (as an end or not!)
- The list allows fast painting of all segments
- Polygons are filled segments
ARCS:
- The dictionary has centers as keys. Each point links to another
dictionary; this one has (width, height) as keys. Those keys in turn
point to every arc having this center and this size.
- The list allows fast painting of all arcs
- Pies are filled arcs