[Impeller] Reland: Use the scissor to limit all draws by clip coverage.

[bdero]

Reland #51698.

This reverts commit 73c145c.

Attempts to improve flutter/flutter#145274.

Our new clipping technique paints walls on the depth buffer "in front" of the Entities that will be affected by said clips. So when an intersect clip is drawn (the common case), the clip will cover the whole framebuffer.

Depth is divvied up such that deeper clips get drawn behind shallower clips, and so many clips actually don't end up drawing depth across the whole framebuffer. However, if the app does a lot of transitioning from a huge clips to a small clips, a lot of unnecessary depth churn occurs (very common in Flutter -- this happens with both the app bar and floating action button in the counter template, for example).

Since progressively deeper layers in the clip coverage stack always subset their parent layers, we can reduce waste for small intersect clips by setting the scissor to the clip coverage rect instead of drawing the clip to the whole screen.

Note that this change does not help much with huge/fullscreen clips.

Also, we could potentially improve this further by computing much stricter bounds. Rather than just using clip coverage for the scissor, we could intersect it with the union of all draws affected by the clip at the cost of a bit more CPU churn per draw. I don't think that's enough juice for the squeeze though.

Before (Play/AiksTest.CanRenderNestedClips/Metal):

Screen.Recording.2024-03-26.at.7.34.29.PM.mov

After (Play/AiksTest.CanRenderNestedClips/Metal):

Screen.Recording.2024-03-26.at.7.38.33.PM.mov

[bdero]

The new golden reproduces the issue that triggered the revert.

The problem was that the MSAA backdrop restore was getting erroneously limited by the clip coverage scissor. The fix is to just draw the restore before replaying the clips.

Before:

Screen.Recording.2024-03-27.at.3.13.55.PM.mov

After:

Screen.Recording.2024-03-27.at.3.14.29.PM.mov

[jonahwilliams]

LGTM

[bdero]

The new golden is missing for some reason. I'm going to push a blank commit to see if it shows up...

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Changes reported for pull request #51731 at sha 638dabb

[jonahwilliams]

At least on the flutter gallery benchmark, we're back to the pre StC baseline:

https://flutter-flutter-perf.skia.org/e/?begin=1709165450&end=1711599083&queries=device_type%3DPixel_7_Pro%26sub_result%3D90th_percentile_gpu_frame_time%26sub_result%3D99th_percentile_gpu_frame_time%26sub_result%3Daverage_gpu_frame_time%26sub_result%3Dworst_gpu_frame_time%26test%3Dnew_gallery_impeller__transition_perf&requestType=0&selected=commit%3D40040%26name%3D%252Carch%253Dintel%252Cbranch%253Dmaster%252Cconfig%253Ddefault%252Cdevice_type%253DPixel_7_Pro%252Cdevice_version%253Dnone%252Chost_type%253Dlinux%252Csub_result%253Dworst_gpu_frame_time%252Ctest%253Dnew_gallery_impeller__transition_perf%252C