Improve robustness of bezier curve intersection

[davidgunderman]

Some robustness issues with the Bezier Curve intersection algorithm:

1. Overlapping curves are not handled yet.
2. Tangent intersections are not found.
3. Curves are considered half-open, intersections at endpoints are not found if they happen at the end of the interval.
4. Issues may arise in cases when control points are concentrated very closely at one endpoint for high order curves.