add back draco files in gulp prepare
add back draco files in gulp prepare

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