cesium 实现卷帘

这里需要ImageryLayer 定义
参考 cesium 图层那些事

```javascript
/**

• 卷帘效果
  */

import {ImageryLayer} from "./ImageryLayer";

export class ShutterEffect {
private map: any;
private _wrapper: any;
private _splitter: any;
private _splitterWidthCenter: any;
private _moveActive: boolean = false;
private _handler: any = null;
private _leftImageryLayer :ImageryLayer|null = null;
private _rightImageryLayer :ImageryLayer|null = null;
constructor(map: any) {
this.map = map;
this.moveHandler = this.moveHandler.bind(this);
}

• start(left:ImageryLayer|null,right?:ImageryLayer|null,splitPosition = 0.5) {
• this.stop();
• if(this._wrapper || this._handler){
• return;
• }
• if(left){
• this._leftImageryLayer = left;
• left.splitDirection(Cesium.ImagerySplitDirection.LEFT);
• }
• if(right){
• this._rightImageryLayer = right;
• right.splitDirection(Cesium.ImagerySplitDirection.RIGHT);
• }
• this.createDom(splitPosition);
• }
•  
• stop() {
• if (this._handler) {
• this._handler.destroy();
• this._handler = null;
• }
• if(this._wrapper){
• this.map.container.removeChild(this._wrapper);
• this._wrapper = null;
• }
• if(this._leftImageryLayer){
• this._leftImageryLayer.removeByMap();
• }
• if(this._rightImageryLayer){
• this._rightImageryLayer.removeByMap();
• }

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