cesium 3d地形和平面地形动态转换，取消3d地形

cesium 关闭和取消3D地形图。

      if(that.isguide){

            console.log('开启3d地形_____')
            //本地的3D地形
            if (window._MapConfig.terrainUrl) {
              console.log("本地的3D地形！")
              viewer.terrainProvider = new Cesium.CesiumTerrainProvider({
                url: window._MapConfig.terrainUrl,
                minimumLevel: 0,
                maximumLevel: 16,
              })
            } else {
              console.log("默认地形-cesium")
              viewer.terrainProvider = Cesium.createWorldTerrain()
            }
            viewer.scene.globe.depthTestAgainstTerrain = true;//是否检查深度


      }else {
            console.log('关闭3d地形_____')
            viewer.terrainProvider = new Cesium.EllipsoidTerrainProvider({});
            viewer.scene.globe.depthTestAgainstTerrain = false;//是否检查深度
      }