[x] Deform a line with circular guides like timing belt.
[ ] Discs can rest on either side of the band.
[ ] Add a hotkey for "pick up" the disc so as to let the user move a disc from one side of the band to the other and/or a hotkey to simply flip which side the disc is on.
[ ] Be sure to allow this curve type to mix with any other curve type closed or open.
[ ] To handle overlapping discs, implement collision detection between inner-to-outer
https://en.wikipedia.org/wiki/Belt_problem https://en.wikipedia.org/wiki/Belt_problem#Pulley_problem http://www.vb-helper.com/howto_net_circle_circle_tangents.html https://en.wikipedia.org/wiki/Tangent_lines_to_circles