[Qt] Canvas arcTo() should draw straight line to p1 if p0, p1 and p2 are collinear

[Qt] Canvas arcTo() should draw straight line to p1 if p0, p1 and p2 are collinear

Reviewed by Simon Hausmann.

The implementation of PathQt's addArcTo() was not float-safe and also had
a case where it drew an 'infinite' line, which is not part of the spec.

http://www.whatwg.org/specs/web-apps/current-work/#dom-context-2d-arcto

We now use qFuzzyCompare() in both cases. The method isPointOnPathBorder()
also had the same problem, and was refactored a bit in the process of fixing
the bug.

Initial patch by Andreas Kling.

https://bugs.webkit.org/show_bug.cgi?id=41412

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@62383 268f45cc-cd09-0410-ab3c-d52691b4dbfc