Attempt to handle tile expiry
Attempt to handle tile expiry

CesiumGS/cesium,CesiumGS/cesium,likangning93/cesium,likangning93/cesium,AnalyticalGraphicsInc/cesium,CesiumGS/cesium,CesiumGS/cesium,likangning93/cesium,likangning93/cesium,likangning93/cesium,CesiumGS/cesium,AnalyticalGraphicsInc/cesium