[Impeller] Reland 3: Implement draw order optimization.

[bdero]

Attempt 1: #54136
Revert 1: #54067
Attempt 2: #54215
Revert 2: #54261
Attempt 3: #54268
Revert 3: #54325

This time for sure!

Includes a fix for the bug discovered in yt.

This turned out to be a bug that was laying dormant in EntityPass for about 5 months. During the clip replay, the clip scissor was getting set to clip_coverage_stack.CurrentClipCoverage() instead of the coverage stored along side the clip being replayed.

Description

For each clip scope, draw opaque items in reverse order and translucent/backdrop-independent items in their original order afterwards. Clips are treated as translucent by the parent scope.

Respects clips, subpass collapse, and the clear color optimization.

Local new_gallery before/after (iPhone 12 mini):

cd ~/projects/flutter/flutter/dev/integration_tests/new_gallery
flutter drive --profile --local-engine-src-path ~/projects/flutter/engine/src --local-engine=ios_profile --local-engine-host=host_profile_arm64 --trace-startup -t test_driver/transitions_perf.dart -d 00008101-000A59A93C10001E

[flar]

Should that read "may be a bug"?

[bdero]

Done.

[jonahwilliams]

As long as the draw order resolver is working off of indices, it should be straightforward to port to the new canvas. That said, I plan to reland the switch after #54604 which would disable this optimization, unless you update exp canvas as well.

[flar]

As long as the draw order resolver is working off of indices, it should be straightforward to port to the new canvas. That said, I plan to reland the switch after #54604 which would disable this optimization, unless you update exp canvas as well.

I'm still trying to reland indices (#54676) so we can't do reorder optimization until that happens. And expCanvas doesn't use indices at all yet, so we'd need to decide if we want to do that conversion as a separate step or combine it with the re-order re-write.

[bdero]

It's hard to know exactly what features we need in the DL until this actually sticks. The nature of what we'd need to do in exp canvas to support this kind of thing changed significantly from the first time I tried to land this.

So I'll help unravel the puzzle after this change sticks.

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Changes reported for pull request #54673 at sha 5ac4d63

[bdero]

This is the only line that changed from the previous attempt (other than the new golden that I added).

clip_coverage_stack.CurrentClipCoverage() changed to next_replay_clip->clip_coverage.