Fixes flutter/flutter#138936

Currently the engine hold a single backbuffer per flutter view layer, which gets swapped with front buffer. This however is too optimistic, as the front buffer in some cases might not get immediately picked up by the compositor. When that happens, during next frame the cache may return a backbuffer that is in fact still being used by the compositor. Rendering to such surface will result in artifacts seen in the video. This seems more likely to happen with busy platform thread. It is also more likely to happen with the presence of "heavy" platform views such as WKWebView.

IOSurface is able to report when it is being used by the compositor (IOSurfaceIsInUse). This PR ensures that the backing store cache never returns surface that is being used by compositor. This may result in transiently keeping more surfaces than before, as the compositor sometimes seems to holds on to the surface longer than it is displayed, but that's preferable outcome to visual glitches.

This PR adds age field to FlutterSurface. When returning buffers to the surface cache (during compositor commit), the age of each surface currently present in cache increases by one, while the newly returned surfaces have their age set to 0.

When returning surfaces from cache, a surface with lowest age that is not in use by compositor is returned.

Surfaces with age that reaches 5 are evicted from the pool. Reaching this age means one of two things:

• When surface is still in use at age 5 it means the compositor is holding on to it much longer than we expect. In this case just forget about the surface, we can't really do anything about it.
• When surface is not in use at age 5, it means it hasn't been removed from cache for 4 subsequent frames. That means the cache is holding more surfaces than needed.

Removing all surfaces from cache after idle period remains unchanged.

Before:

After:

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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