Move `copyPixelsFromBuffer` to a background thread.

[0xZOne]

Move the time-consuming copyPixelsFromBuffer function to a background thread to avoid blocking the UI thread. At the same time, remove the unnecessary currentImage member.

Pre-launch Checklist

• [x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [x] I read the Tree Hygiene wiki page, which explains my responsibilities.
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• [ ] I listed at least one issue that this PR fixes in the description above.
• [x] I added new tests to check the change I am making or feature I am adding, or the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• [ ] I updated/added relevant documentation (doc comments with ///).
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• [x] All existing and new tests are passing.

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[flutter-dashboard[bot]]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact "@test-exemption-reviewer" in the #hackers channel in Chat (don't just cc them here, they won't see it! Use Discord!).

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[flutter-dashboard[bot]]

This pull request has been changed to a draft. The currently pending flutter-gold status will not be able to resolve until a new commit is pushed or the change is marked ready for review again.

[johnmccutchan]

This PR is missing a detailed description of the changes and benchmarks associated with.

[0xZOne]

This PR is missing a detailed description of the changes and benchmarks associated with.

I'm not quite clear on how to include a benchmark. Would you mind offering some guidance? Thanks~

[johnmccutchan]

This PR is missing a detailed description of the changes and benchmarks associated with.

I'm not quite clear on how to include a benchmark. Would you mind offering some guidance? Thanks~

Since your making this change for performance reasons. I suggest you write a small program that allows you to measure/quantify how much time you save on the main UI thread? That is, a program that calls convertImageToBitmap with a bunch of different image sizes and measures how long it takes to complete.

That will give us an understanding of how impactful this change is.

[jonahwilliams]

We don't think this patch is safe to land, and lacking the benchmarks or evidence that it is faster and correct we don't plan to continue reviewing this.