Add test for ViewportQuad
Add test for ViewportQuad

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