added flag for enabling/disabling shadows fading when the light source is close to the horizon
added flag for enabling/disabling shadows fading when the light source is close to the horizon

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