[DONT LAND][ios][platform_view]round in layers and round out platform views

[hellohuanlin]

The previous PR flutter/flutter#143420 rounds out the layers and rounds in the platform views. This results in missing pixel on the edge of the intersection when there's fractional coordinate (as shown in the screenshot below), because platform view is below the layers.

This PR rounds in the layers, and rounds out the platform views, so that the edge pixels of the intersection are included in the overlay.

Note that the edge pixel of the layers will give the ownership to platform view. This is just a single (or partial) pixel and will likely be unnoticeable.

List which issues are fixed by this PR. You must list at least one issue.

flutter/flutter#143420

If you had to change anything in the flutter/tests repo, include a link to the migration guide as per the breaking change policy.

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 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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[hellohuanlin]

closing this in favor of #51687