- Draw also a folding template for the flexagon.
- Complete the implementation of the diagram backends.
- Revisit the design of the Diagram abstraction, and maybe split it to
  a separate project.
- Show also the faces with the "wrong" tile orientation, to make it easier to
  build flexagons with a multi-symmetric layout, like shown in
  https://www.youtube.com/watch?v=NBJrGbbxceU