BMFTile constructor
- {@required BMFCoordinateBounds visibleMapBounds,
 - @required BMFTileLoadType tileLoadType,
 - int maxZoom,
 - int minZoom,
 - int maxTileTmp,
 - String url}
 
BMFTile构造方法
Implementation
BMFTile(
    {@required this.visibleMapBounds,
    @required this.tileLoadType,
    this.maxZoom,
    this.minZoom,
    this.maxTileTmp,
    this.url});