Invert relationship between velocity and stroke width

According to the original article (and real-world experience) the more velocity, the thinner the stroke should be (because ink doesn’t have as much time to deposit on the paper).

I think that thus modified, the curves appear more natural.
Merge branch 'patch-1' of https://github.com/victor/SignatureDemo into victor-patch-1
Fix dots after inverting velocity
Merge branch 'victor-patch-1'