makeViewMatrix function
Constructs a new OpenGL view matrix.
cameraPosition specifies the position of the camera.
cameraFocusPosition specifies the position the camera is focused on.
upDirection specifies the direction of the up vector (usually, +Y).
Implementation
Matrix4 makeViewMatrix(
    Vector3 cameraPosition, Vector3 cameraFocusPosition, Vector3 upDirection) {
  final r = Matrix4.zero();
  setViewMatrix(r, cameraPosition, cameraFocusPosition, upDirection);
  return r;
}