Given a list of vertices and faces, generate the normals for a triangle mesh.
Geared more towards "soft" low-poly normals, as you can't use indexed draw calls with the resulting normals. If you're just planning on straight smooth vertex normals, use the normals module. If you want low-poly and only have a "triangle soup" mesh, face-normals will still work for you.
cells is an indexed list of faces. Each face should be three elements long,
one element per point.
positions is a list of vertex positions.
flatness is a value between 0 and 1, where 0 will return results only from
the vertex normals and 1 will return results only from the face normals. 0.5
will be the middle ground between them.
Optionally, you can pass in an array-like output object to set the values
directly. If not, a Float32Array will be created for you - either way, the
new/updated array will be returned.