Auto merge of #24464 - rasviitanen:offscreen-canvas, r=jdm

fix getimagedata returns empty pixels

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GetImageData for OffscreenCanvas without an associated canvas element returned blank pixels. To solve this, we now pass a `Size2D` instead of a canvas to relevant functions.

I don't quite know if it's ok that `OffscreenCanvasRenderingContext2D` now have a `width` and `height`, but I found no other reasonable solution to this.

There are some tests that previously were marked as `PASS` that are now failing. It seems that they were passing for the wrong reason.
These are:
> tests/wpt/metadata/offscreen-canvas/pixel-manipulation/2d.imageData.put.unchanged.html.ini
> tests/wpt/metadata/offscreen-canvas/pixel-manipulatio/2d.imageData.put.unchanged.worker.js.ini
> tests/wpt/metadata/offscreen-canvas/the-offscreen-canvas/initial.reset.path.html.ini
> tests/wpt/metadata/offscreen-canvas/the-offscreen-canvas/initial.reset.path.worker.js.ini
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #24271 (GitHub issue number if applicable)

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- [x] There are tests for these changes

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