Add translucency rectangle to sandcastle example [skip ci]
Add translucency rectangle to sandcastle example [skip ci]

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