Jelle Raaijmakers 5865cf5864 LibWeb: Use bitmap's alpha type instead of assuming unpremultiplied ...

When converting a `Gfx::Bitmap` to a Skia bitmap, we cannot assume the
color data is unpremultiplied. For example, everything canvas-related
uses premultiplied color data:

  https://html.spec.whatwg.org/multipage/canvas.html#premultiplied-alpha-and-the-2d-rendering-context

We were probably assuming unpremultiplied since that is what the PNG
decoder gives us. Since we now make `Gfx::Bitmap` identify what alpha
type is being used, we can instruct Skia a bit better :^)

Update our `EdgeFlagPathRasterizer` to use premultiplied alpha instead
of unpremultiplied so we can apply alpha correctly for path masks.

This fixes the dark borders sometimes visible when SVGs are blended
with a colored background.

This also exposed an issue with our `CanvasRenderingContext2D`, which is
supposed to hold a bitmap with premultiplied alpha internally but expose
a bitmap with unpremultiplied alpha in `CanvasImageData`. Expand our C2D
test to include the alpha channel as well.

Finally, this also exposed an off-by-one issue in
`EdgeFlagPathRasterizer` which caused the last scanlines for edges to
render incorrectly. We had some reference images which included these
corruptions (they were almost unnoticeable), so update them as well.

2024-08-07 18:51:12 +02:00

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