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

[itfsw]

Sandcastle example: Example 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

viewer.flyTo(xxx).when(()=>{
    viewer._zoomIsFlight  = false
})

before use destroy, it still cannot free entity object，so it may happend when use flyTo method