[Multiwindow] Backing store and compositor

[dkwingsmt]

This PR is based on https://github.com/flutter/engine/pull/36392 and currently contains parent PRs . See https://github.com/dkwingsmt/engine/pull/8 for net changes.

This PR turns backing store and compositor to fitting the multi-view world. The change stops at:

• Rasterizer::SubmitFrame, where it will expect a view ID from LayerTree but uses a fake one for now.
• macos/FlutterEngine, where it should pick a viewController based on surface ID, but it only has one view controller for now.

This PR makes non-trivial changes to the embedder API:

• A new function AddRenderSurface.
• Deprecates FlutterCompositor.present_layers_callback, adds FlutterCompositor.present_layers_view_callback.
• Adds FlutterBackingStoreConfig.view_id.

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.

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