Allow multiple rounds of shader warm ups

[liyuqian]

Description

This is required for a high-priority Google client to remove its two final shader compilation jank. I tried to not modify any of the existing engine or framework API so this can be rolled quickly without manual fixes.

Related Issues

#813

Tests

Ask for preliminary code review now to accelerate the process. Will add a flutter drive performance benchmarks similar to cubic_bezier_perf__timeline_summary soon.

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[dnfield]

I'm having trouble seeing why this belongs in the framework. An app developer doesn't benefit much from this implementation, and we already know it's a less than ideal implementation (using screenshots instead of flushes) of an API that already seems difficult to reason about using correctly.

As written, this seems like something that could easily be a package (or just written directly in an app).

If this really is the fix that the internal customer needs, perhaps we could just implement it in their app rather than the framework. I strongly suspect we'd have a better time even doing this in their embedder rather than in the Dart code, since we could presumably fire this off asynchronously as soon as Skia is loaded rather than waiting for the Dart VM to arrive here.

[liyuqian]

Thanks @dnfield for the suggestion! I've verified that it's indeed possible to fix the problem without any Flutter framework or engine side changes. The downside is that there will be two screenshots. But it's Ok as we have several seconds of budget. Close this PR, and let's move the the Google-internal changes.