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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.