code: function tileRsys (x, y, z) {
      let max = Math.pow(2, z);
      return { scale: 1 / max, center: { x: 2 * (x + 0.5) / max - 1, y: 1 - 2 * (y + 0.5) / max } };
  }
docs: RSys of a tile (mapping local tile coordinates in +/-1 to NWMC)
  @param {number} x - TC x coordinate
  @param {number} y - TC y coordinate
  @param {number} z - Tile zoom level
  @return {RSys}