Feature: 想要实现地图的导出，需要一个Scene上所有layer 上render 结束的回调

目的： 导出Scene所有图层，需要等待所有图层绘制完成，目前Scene上没有这样的事件回调函数。
期望：Scene提供一个Completed的一个事件回调，这个时机可能保证所有的layer图层绘制已结束。

The issue requests a Scene-level Completed callback for exporting all layers after rendering finishes, but names no files, tests, or entry points. Start by tracing the Scene rendering lifecycle and existing layer or render callbacks; done should provide a reliable completion signal after every Scene layer has finished drawing.

Written by the indexing model from the issue text.