Blend issues with impeller about flutter-wonderous-app HOT 6 CLOSED

cc @bdero

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Fix: flutter/engine#36338.

BTW, that overhang shadow isn't missing in Impeller, right? I assume it fades in when starting to scroll down.

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This looks like the alpha channel is either getting dropped or ignored somewhere along the line for these images. Trying to reproduce this.

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I believe the issue is the way I did the alpha compositing for advanced blends in the case where a foreground color is specified. In this case, the source color is being overridden to opaque black, so the fragment alpha output is always 1: https://github.com/flutter/engine/blob/main/impeller/entity/shaders/blending/advanced_blend.glsl#L43

I'm currently working out the correct behavior for this.

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Correct, the shadow is good w/ impeller, its just not visible in that screenshot.

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Fixed in 2e0d6f100c9ea77d2b31b04752fa4e185b1a05cf

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