Breaking changes: Box and Triangle types replaced with gonum types

[soypat]

Breaking changes are incoming as soon as gonum/gonum#1791 is resolved. This would replace the d3.Box, render.Triangle3 and d2.Box types with their respective gonum types.

Box change

This change would allow users to work with the Box types freely in their code. As of now the Box types are hidden behind an internal wall which makes them invalid for use in external code.

Triangle change

The Triangle type would further decouple the render package and make the Renderer interface more flexible and dependable as the Triangle type would be tied to a solid, trusted and tested library.