Get url and rect for max level tile to load greater than max level from source, had to load from max level. 因为瓦片数据并未覆盖所有级别瓦片，如MapBox地形瓦片最高只到15级，如果要显示18级以上瓦片，不能从17级瓦片中获取，只能从15级瓦片里截取一部分
max tile url and bounds in in maxTile
Get url and rect for max level tile to load greater than max level from source, had to load from max level. 因为瓦片数据并未覆盖所有级别瓦片，如MapBox地形瓦片最高只到15级，如果要显示18级以上瓦片，不能从17级瓦片中获取，只能从15级瓦片里截取一部分