Improve and simplify handling of start segments.
Improve and simplify handling of start segments.

Removing findStartSegment() and merging functionality into findBestIntersection() leads to simpler code and solves some remaining issue, e.g. https://github.com/paperjs/paper.js/issues/784#issuecomment-153282113

iconexperience/paper.js,lehni/paper.js,iconexperience/paper.js,lehni/paper.js,iconexperience/paper.js,lehni/paper.js