[Impeller] Fix and reland drawPaint collapsing optimization.

[gaaclarke]

Collapses DrawPaint calls into clear colors when possible.

issue flutter/flutter#129292
relands #41711

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[bdero]

LGTM!

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Changes reported for pull request #43097 at sha 7848976

[gaaclarke]

I checked the golden image results:

1. CanRenderOffscreenCheckerboard - This doesn't reproduce locally, it looks like the save layer has a color that isn't transparent. I think we need to conditionally set the clear color instead of always setting it.
2. CanRenderDestructiveSaveLayer - This is drawing wrong too, the correct drawing does not have a red background. Well have to figure out why that isn't blending correctly.

[gaaclarke]

For the CanRenderOffscreenCheckerboard problem it was because when we call EntityPass::SetClearColor, that clear color was used as the clear color for every subpass in an entity pass. That test uses a multiply blend mode so the cream color would get multiplied, creating the error. I've updated it so that when you call EntityPass::SetClearColor, the clear color only applies to the offscreen_target. After typing this out though, this might fix Vulkan and break Metal since it isn't going down the same path. We'll see.

[gaaclarke]

CanRenderDestructiveSaveLayer should be fixed by the same fix. It's difficult to know for sure, I think Skia Gold draws transparent black as white and locally we draw it as black.

[auto-submit]

auto label is removed for flutter/engine, pr: 43097, due to - The status or check suite Linux linux_web_engine has failed. Please fix the issues identified (or deflake) before re-applying this label.