Function edge_function

crates/luacad/src/render.rs:736–738

(a: [f32; 3], b: [f32; 3], c: [f32; 3])

        }
        n[vi] = normalize(accum);
      }
      n
    } else {
      let fn_ = face_normals[fi];
      [fn_, fn_, fn_]
    };