[embedder][fuchsia] Add paint regions to the FlutterLayer struct

[abrush21]

This change adds an array of FlutterRects to the FlutterLayer struct in the embedder API. These rects represent the regions within the layer that contain painted content. The fuchsia embedding will report this information to the platform compositor, to ensure that the hit test behavior for a particular layer is consistent with its visual geometry.

fxbug.dev/106560

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.
• [X] I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• [X] 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 Hixie said the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• [X] I updated/added relevant documentation (doc comments with ///).
• [X] I signed the CLA.
• [X] All existing and new tests are passing.

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[abrush21]

The API itself looks fine to me.

Is there anything missing from this change, or is it good to go as-is?

[chinmaygarde]

The implementation as well as the updates to the unit-test fixture along with unittests should go here?

[Hixie]

@abrush21 Is this still on your radar?