Fatline

Almost complete rewrite of the bezier fatline clipping routines.

- It avoids some issues the earlier code had —premature branching to split-and-recurse, where iteration was enough
- Shorter, simpler and faster, than the code it replaces.

Tested against Mathematica's root finder.

/Hari

PS  @lehni The new code is very accurate, We don't need these hacks(https://github.com/paperjs/paper.js/commit/492e78df7dcdb224450e1f76c910fcd99246afbf) anymore!