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.
