function convertSTtoUV(s) {
  if (s >= 0.5) {
    return (1 / 3) * (4 * s * s - 1);
  }
  return (1 / 3) * (1 - 4 * (1 - s) * (1 - s));
}
S2 provides 3 methods for the non-linear transform: linear, quadratic and tangential.
This implementation uses the quadratic method because it provides a good balance of
accuracy and speed.

For a more detailed comparison of these transform methods, see
{@link https://github.com/google/s2geometry/blob/0c4c460bdfe696da303641771f9def900b3e440f/src/s2/s2metrics.cc}
@private