[Flutter Web(HTML)] fix: shader mask is painted incorrectly on shared offscreen canvas

[Kingtous]

Hi from Dora team, which powers web developers to build their 3d websites in just a few seconds.

This PR fixes flutter/flutter#133134.

The size of the shared canvas should not only be updated if both width and height are not the same.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said 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 ///).
• I signed the CLA.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[flutter-dashboard]

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 Hixie on the #hackers channel in Chat (don't just cc him here, he won't see it! He's on Discord!).

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[flutter-dashboard]

Golden file changes have been found for this pull request. Click here to view and triage (e.g. because this is an intentional change).

If you are still iterating on this change and are not ready to resolve the images on the Flutter Gold dashboard, consider marking this PR as a draft pull request above. You will still be able to view image results on the dashboard, commenting will be silenced, and the check will not try to resolve itself until marked ready for review.

Changes reported for pull request #44998 at sha a1b0a38

[Kingtous]

Any updates here? :)

[wanjm]

Any updates here? :)
maybe you should ask for reviewer.

[flutter-dashboard]

This pull request executed golden file tests, but it has not been updated in a while (20+ days). Test results from Gold expire after as many days, so this pull request will need to be updated with a fresh commit in order to get results from Gold.

[harryterkelsen]

The code LGTM. Can you please rebase on main so we can get up-to-date goldens on this PR? Thanks!

[harryterkelsen]

Please rebase on main

[flutter-dashboard]

Golden file changes are available for triage from new commit, Click here to view.

Changes reported for pull request #44998 at sha 4cd4554

[flutter-dashboard]

Golden file changes are available for triage from new commit, Click here to view.

Changes reported for pull request #44998 at sha 5b17cc6