Update DrawPolygon.js

clone一份数据，PolygonHierarchy会修改this._positions原始数据，导致_stopdHook转换之后的高度异常

modules/plot/draw/DrawPolygon.js

@@ -34,7 +34,9 @@ class DrawPolygon extends Draw {
         ...this._style,
         hierarchy: new Cesium.CallbackProperty(() => {
           if (this._positions.length > 2) {
-            return new Cesium.PolygonHierarchy(this._positions)
+            return new Cesium.PolygonHierarchy(
+              this._positions.map(item => item.clone())
+            )
           } else {
             return null
           }