Improve cubic curve offset computation

[bourgesl]

In some case, offsetted curve may have a loop because solving the 2nd degree equation is not stable in Stroker.computeOffsetCubic().

Using twice the solution in computeOffsetQuad() based on segment intersection, is a good enough approach to make offsets stable

[bourgesl]

See https://github.com/bourgesl/marlin-renderer/blob/better-curves/src/main/java/org/marlin/pisces/DStroker.java 

[bourgesl]

Still WIP in branch better-curves

[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/