Improve accuracy of Helper.cubicRoots()

[bourgesl]

1. Use final newton root refinement in cubic roots solve to refine accuracy
2. For clip intersections (xPoints, yPoints), ensure x(t) or y(t) are outside of the clip (side issue) => extra clip padding will then be useless (1.0 for float, 0.25 for double variants) as roots do not ensure enough precision on the point coordinates for now

[bourgesl]

Really worth to improve 2nd and 3rd order polynom solver in Marlin-math:
https://github.com/bourgesl/marlin-math

[bourgesl]

To fix marlin's bezier quad or cubic issues, I found how to solve accurately:

• quadratics: http://people.eecs.berkeley.edu/~wkahan/Qdrtcs.pdf
• cubics: http://people.eecs.berkeley.edu/~wkahan/Math128/Cubic.pdf

So happy !

See
http://people.eecs.berkeley.edu/~wkahan/Math128/