add ClippingPolygon

src/index.js

 }
 
 export function ready(options = {}) {
-  if(__isInitialized){
-     return Promise.resolve()
+  if (__isInitialized) {
+    return Promise.resolve()
   }
   __cmdOut && __cmdOut()
   if (options['baseUrl']) {

src/modules/third-part/index.js

     Cesium.createGooglePhotorealistic3DTileset,
   createElevationBandMaterial: Cesium.createElevationBandMaterial,
   ModelAnimationLoop: Cesium.ModelAnimationLoop,
+  ClippingPlane: Cesium.ClippingPlane,
+  ClippingPlaneCollection: Cesium.ClippingPlaneCollection,
+  ClippingPolygon: Cesium.ClippingPolygon,
+  ClippingPolygonCollection: Cesium.ClippingPolygonCollection,
 }