addVertexToFace2 method
Implementation
ConvexHull addVertexToFace2(VertexNode vertex, Face2 face) {
  vertex.face = face;
  if (face.outside == null) {
    assigned.append(vertex);
  } else {
    assigned.insertBefore(face.outside!, vertex);
  }
  face.outside = vertex;
  return this;
}