Cleanup iframe first to prevent memory leaks, see #1609

[yuyuyzl]

Summary

This PR fixes/implements the following bugs/features

• [x] Fixed memory leaks caused by cloned iframe's detached window

I'm working on a PNG sequence rendering project, and I was trying to use this for lightweight rendering (heavyweight ones should use headless chrome based project like Remotion)

About 200 frames can be rendered well, and then the page got out ouf memory and crashed.

With a heap snapshot, it is clear that "Detached window" consumes most of the memory.

Tried to clean up iframes and remove it manually in the promise resolve, it works very well, the memory usage of Chrome barely increases after that, and I can render 2000+ frames at once.

See #1609 for the detailed cleanup code.

Test plan (required)

You may need a heavy dom tree with some datasrc images to speedup the memory leaks. Then, call html2canvas repeatedly, monitor Chrome's memory usage in taskmgr, or until it crashes.

Code formatting

Please make sure that code adheres to the project code formatting. Running npm run format will automatically format your code correctly.

Sure.

Closing issues

Fixes #1609

[Sharcoux]

Can be tested in @cantoo/html2canvas