Url Template
Slippy-map URL template with the WorldWind dialect of substitution tokens:
{x}/{y}/{z}— tile coordinates.{lang}— current locale language code.{rand=a,b,c}— random choice from the supplied comma-separated values.{i}—x % 4 + y % 4 * 4, the 16-server tile-hash pattern some XYZ providers use.
Parses the template once at construction; buildUrl is cheap to call per tile.