[Impeller] Fix alpha management issues for advanced blends. (#50070)

Resolves https://github.com/flutter/flutter/issues/140970.

The issue is easy to miss in the goldens if you don't know what to look for, but it was always present in one way or another. In the "before" image below, we end up removing some of the backdrop alpha as a result of the blend, which should never happen with an advanced blend:

<img width="171" alt="image" src="https://github.com/flutter/engine/assets/919017/b0ac7c00-fba7-4c86-82c9-94b99ea400e3">

This was happening because we were lerping towards the original source alpha from the destination alpha. So in the sad smiley case, the RGB getting written to the framebuffer was exactly correct, but the alpha would end up being less than 1, which the window manager then blends against the black-initialized framebuffer.

Blending with the black background just so happened to perfectly match the appearance of the color being double-premultiplied, which is why increasing the source color's vibrancy by unpremultiplying also looks correct. But again, that only works iff the pass texture winds up being drawn to a black background, which is a rare case for offscreen passes.

Skia (Flutter documentation):
![image](https://github.com/flutter/engine/assets/919017/aeb2a7cf-50a4-460d-9247-054a86833018)

Impeller before:
<img width="398" alt="image" src="https://github.com/flutter/engine/assets/919017/fda958c5-1bd6-4272-a2b4-f8e25c42cb2f">

Impeller after:
<img width="398" alt="image" src="https://github.com/flutter/engine/assets/919017/a4ca51f8-c287-4e27-9979-44b1c91302b2">