Some improvements

[BenWheatley]

Summary of all changes on my branch:

1. Remove commented-out code
2. Remove code I'm surprised even runs without crashing (c.strokeW just by itself on one line)
3. Remove unused and nonfunctional code
4. Add some {}s, to avoid a certain category of bug, including in one case where I had to guess the correct behaviour (scene.frameNr++; when paused?)
5. Speed improvements: cache index calculations, move invariants out of loops — feel free to disregard these if you feel they detract from the educational value of the project