when we use viewer.destroy after viewer.flyTo(some entity or dataSource)，will cause memory leaks

Sandcastle example: [Example](https://sandcastle.cesium.com/#c=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)
Browser: Chrome
1. when use zoomTo will never case;
2. use flyTo will like this:
3. we guess something happened at scene.postRender -> updateZoomTarget -> viewer._zoomIsFlight -> camera.flyToBoundingSphere -> cancel when viewer.destroy
so we try
```javascript
viewer.flyTo(xxx).when(()=>{
viewer._zoomIsFlight = false
})
```
before use destroy, it still cannot free entity object，so it may happend when use flyTo method

Start with the linked Sandcastle example in Chrome and reproduce the difference between zoomTo and flyTo before viewer.destroy. Trace the issue's stated path from scene.postRender through updateZoomTarget and viewer._zoomIsFlight to camera.flyToBoundingSphere, then verify that destroying the viewer releases the entity object without requiring the proposed internal flag change.

Written by the indexing model from the issue text.