added a variable to help determine if heading is changed
added a variable to help determine if heading is changed

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