[reland][web] Refactor renderers to use the same frontend code #174588

[harryterkelsen]

Refactors the renderer code so both renderers (Skwasm and CanvasKit) use the same SceneBuilder and platform view embedding code. This change is discussed in a design doc here: https://flutter.dev/go/web-renderer-unification

Fixes #172311
Fixes #172308
Fixes #142072

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
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• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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Code Review

This pull request introduces a major refactoring to unify the rendering code for the Skwasm and CanvasKit web renderers. The changes are extensive, moving shared logic into a common Renderer base class and abstracting backend-specifics into Rasterizer implementations. This unification also involves removing old scene building logic and replacing it with a new shared implementation. Additionally, the testing infrastructure is updated to support different build modes. The changes are well-structured and consistent with the goal of renderer unification. A positive side-effect of this work is that many tests previously skipped for Skwasm are now enabled, indicating better feature parity. The overall code quality is high, and I found no issues in this large but well-executed refactoring.

[harryterkelsen]

@mdebbar PTAL

This is a reland of #174588 with some tweaks to fix the bugs that were discovered which caused us to revert the original PR.

The changes from the original PR are small. They can be found in bench_mouse_region_grid_hover.dart and bench_mouse_region_grid_mixed_hover.dart which fixes the benchmark crash. The benchmark crashed due to Skwasm speeding up due to using transferToImageBitmap instead of createImageBitmap, which caused timing issues with postFrameCallbacks being run after the benchmark was stopped. The benchmarks, as they were written before, were buggy.

There is another change to address a crash which was discovered in Google3 which happens if a frame is pumped with an empty viewport, which causes the renderer to early-exit out of rendering, but without populating the fields in the FrameTimingRecorder, which caused a crash when we tried to submit timings. This is fixed in rasterizer.dart and renderer.dart by having Rasterizer.draw return a bool indicating if it actually drew the frame and only recording frame timings if the frame was actually drawn.

[mdebbar]

Looks great! I hope it sticks this time 🤞