Add TrianglesGeometry

[DFU398]

Adds TrianglesGeometry as a new type of geometry that can be used for Mesh objects.
Usage:

scene.Add(
    new Mesh()
    {
        Geometry = new TrianglesGeometry()
        {
            Triangles = new List<Triangle3>()
            {
                // ...your triangles here...
            }
        }
    }
);