PlatformView partial blur

[cyanglaz]

This PR makes the PlatformView able to be partially blurred based on the paint_bounds. Right now the rect is hard-coded, I still need to wire up the rect from backdrop_filter_layer. My idea is to have a PlatformViewFilterView for each backdrop_filter_layer and each PlatformViewFilterView has its own blurRadius and frame.

This is currently a draft, feel free to fork from here and finish it!

Fixes: https://github.com/flutter/flutter/issues/50183

Things to do in this PR:

1. Add logic where each PlatformViewFilterView maintains a PlatformViewFilter.
2. For each one blue filter, add a PlatformViewFilterView as a subview of ChildClippingView.

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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