[Impeller] Support YUV external textures on iOS

[ColdPaleLight]

Based on https://github.com/flutter/engine/pull/36498

new code https://github.com/flutter/engine/pull/36685/commits/da611f656433e866f07c74a9469e56f73cbd1ee0

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.

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