Auto merge of #16238 - gterzian:implement_update_the_image_data, r=jdm

Implement "update the image data"

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Spec compliant implementation of the [update the image data algorithm](https://html.spec.whatwg.org/multipage/embedded-content.html#update-the-image-data).

Currently still a work in progress, the ['async src complete test`](https://github.com/servo/servo/blob/master/tests/wpt/web-platform-tests/html/semantics/embedded-content/the-img-element/img.complete.html#L33) is still passing as it was before, even though I switched to the new code, so I guess that's something.

@jdm I will be picking this up next weekend, I left a bunch of TODO and NOTES in the code, if you or someone else have time this week I would appreciate an initial scan and feedback.

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- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #11517 (github issue number if applicable).

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