function getKey(x, y, level) {
  return `${x}-${y}-${level}`;
}
Checks if the next interval is approaching and will start preload the tile if necessary. Otherwise it will
just add the tile to a list to preload when we approach the next interval.

@param {number} x The tile X coordinate.
@param {number} y The tile Y coordinate.
@param {number} level The tile level.
@param {Request} [request] The request object. Intended for internal use only.