[web] Add test that pictures are not rasterized when clipped out

[harryterkelsen]

Adds a test that checks that pictures are not composited if they are clipped out (or not in the frame) in the final scene. This is important to test since we have benchmarks that depend on this behavior being implemented properly.

Part of the web renderer unification. See #172311

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, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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

Note: The Flutter team is currently trialing the use of Gemini Code Assist for GitHub. Comments from the gemini-code-assist bot should not be taken as authoritative feedback from the Flutter team. If you find its comments useful you can update your code accordingly, but if you are unsure or disagree with the feedback, please feel free to wait for a Flutter team member's review for guidance on which automated comments should be addressed.

[fluttergithubbot]

An existing Git SHA, f5a8f4ba77167a80e05ab0428157e05de5c60370, was detected, and no actions were taken.

To re-trigger presubmits after closing or re-opeing a PR, or pushing a HEAD commit (i.e. with --force) that already was pushed before, push a blank commit (git commit --allow-empty -m "Trigger Build") or rebase to continue.

[gemini-code-assist]

Code Review

This pull request adds a new test file to verify that pictures are not rasterized when they are clipped out of the scene. This is achieved by adding a @visibleForTesting getter to expose the EmbedderFrameContext for inspection in tests. The tests are well-structured and cover various culling scenarios. My feedback focuses on improving the maintainability of one of the tests by refactoring duplicated code.