Refactor ImageCache::find_image_or_metadata -> ImageCache::{get_image, track_image}
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Updated the `ImageCache` trait to replace `find_image_or_metadata` with two new functions `track_image` and `get_image`, as well as a new enum (`ImageCacheResult`).
As a result, I was able to refactor the functions that previously called `find_image_or_metadata` pretty cleanly. For a list of these functions, please see the commit information.
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- [x] These changes fix#21289 (GitHub issue number if applicable)
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This safe method is the basic block to access element attributes from layout.
We reuse it in the other attr-related layout methods to remove a pretty big
source of rampant unsafe code between script and layout.
There are a few canvas2d-related dependencies that haven't updated, but they
only use euclid internally so that's not blocking landing the rest of the
changes.
Given the size of this patch, I think it's useful to get this landed as-is.
