Fix pickTranslucentDepth for ground primitives
Fix pickTranslucentDepth for ground primitives

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