add(triangle): simple triangle subdivision

[gsimone]

See

https://graphics.stanford.edu/~mdfisher/subdivision.html
https://tylerxhobbs.com/essays/2017/aesthetically-pleasing-triangle-subdivision

Splitting the triangle in two:
- get a random vertex A
- get the mid point of the opposing side M
- add a new edge AM
- return 2 new triangles ( ABM, AMC )

Splitting in 4 (sudbidivision):
- get midpoint of each vertex ( D, E, F )
- return 4 new triangles [ ABF, DBE, ECF, DEF ]