fixed Curve::Intersect() about z component.

[KojiNakamaru]

This fix introduces a subexpression about z as described in Errata (http://eaflux.com/index.html.en):

Page 2, bottom right: If control points are placed non-uniformly in ray direction, the resulting v parameter may have a large margin of error because the algorithm computes v in 2D space. To compensate this, we should add a subexpression about z in the maximum depth computation. (Thanks to Emil Kirichev)

Anyway, thank you very much for implementing the algorithm!

[mmp]

Thanks!!