Auto merge of #20680 - Brody-Eastwood:master, r=emilio

Moved Canvas rendering to a single thread.

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Implements the "Subsequent Steps" part of the [NCSU Canvas rendering project.](https://github.com/servo/servo/wiki/Canvas-rendering-project)

I moved most of the functionality from CanvasPaintThread to CanvasData, so CanvasPaintThread essentially just pulls the info out of the message and calls a method on a particular CanvasData element.

I ran into some awkwardness with the fact that one can only take a single mutable reference from a hashmap, though. DrawImageInOther is not really possible to do with only one reference at a time, so it awkwardly still lives in CanvasPaintThread, basically. I also would've preferred to take the reference at the top as soon as I get the CanvasId, since that looks cleaner than having them all start with "canvas_paint_thread.canvases.get_mut[&canvas_id].unwrap()" but that makes trying to take the second reference for DrawImageInOther fail to compile. I'm definitely open to suggestions on how to make that less gross.

The timed single-canvas drawing improved in performance from around ~2.2ms to around ~1.7ms. Slither.io runs better and doesn't crash, but I'm not having it crash on my copy from before these changes, so I don't know if that's new behavior or not.

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- [X] `./mach build-geckolib` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #13879 and #10381.

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- [X] There are tests for these changes (added in the initial steps)

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