Reland: Avoid a copy in EncodeImage

[zanderso]

Description

While working on a hobby project, I noticed that image.toByteData(format: ui.ImageByteFormat.png) is quite slow for large images (>4s for a 1080p image on a Nexus 5x). Most of that work is not on the Dart thread, but the final copy into a Dart heap Uint8List appeared to be, and was taking ~1.5s.

Instead of doing that last copy, this PR creates an external typed data object instead.

This is a reland of #19504. The buffer is not allocated in read-only memory, skia will not write it, and it is owned by Dart on return, so it does not need to be an UnmodifiableByteDataView. It can still be Uint8List.

Tests

I added the following tests:

image_encoding_unittests.cc

Breaking Change

Did any tests fail when you ran them? Please read [handling breaking changes].

• No, no existing tests failed, so this is not a breaking change.

[liyuqian]

This seems to be a big improvement similar to #18838 (this PR seems to speed up the image exporting while #18838 speeds up the image importing). I wonder if we could have a benchmark in the engine similar to #18945 to quantify this improvement?

[zanderso]

This seems to be a big improvement similar to #18838 (this PR seems to speed up the image exporting while #18838 speeds up the image importing). I wonder if we could have a benchmark in the engine similar to #18945 to quantify this improvement?

I tried this, setting up an isolate as in #18945, but hit a crash in Picture::RasterizeToImage, presumably because more setup is required. @liyuqian are there any examples of benchmarks that do something like that?

[liyuqian]

@zanderso : you may need to setup the whole shell in order to benchmark Picture::RasterizeToImage. An example of setting up shell in benchmarks could be found in https://github.com/flutter/engine/blob/9398bc4b1f4c4d0d8490df31a7c5ae02039067e8/shell/common/shell_benchmarks.cc#L56

You may also be able to utilize the ShellTest fixture to create the shell and run the engine.
https://github.com/flutter/engine/blob/d3bc43e74e2e94582061dbd83fab0a9ce8d31a5c/shell/common/persistent_cache_unittests.cc#L263

Once the shell is properly set up, you should be able to call Picture::RasterizeToImage on the UI thread using

shell->GetTaskRunners().GetUITaskRunner()->PostTask(...)

[zanderso]

Re-run of a flaky Fuchsia job succeeded, so I will land this on red to get things going again.

I have a benchmark for this running locally, but I will submit a separate PR for it to keep this one from getting too big.