This changes Gfx::ScalingMode to reflect the three modes of scaling we
support using Skia, which makes it a bit easier to reason about the mode
to select. New is ::BilinearMipmap, which uses linear interpolation
between mipmap levels to produce higher quality downscaled images.
The cubic resampling options Mitchell and its sibling CatmullRom both
produced weird artifacts or resulted in a worse quality than
BilinearMipmap when downscaling. We might not have been using these
correctly, but the new ::BilinearMipmap method seems to mirror what
Chrome uses for downscaled images.
We already had the API, but drawing to the canvas was not affected by
any created CanvasPattern. This moves CanvasPatternPaintStyle to LibGfx
so we don't have to reach into LibWeb, and implements the plumbing to
let Skia use images as a fill pattern.
This is implemented by these related changes:
* The Skia alpha type 'Opaque' is selected for surfaces that were
created with the intention of not having an alpha channel.
Previously we were simply creating one with alpha.
* Clearing now happens through Skia's `clear()` which always uses the
source color's value for the result, instead of setting all values
to 0.
* CanvasRenderingContext2D selects a different clearing color based on
the `alpha` context attribute's value.
CSS filters work similarly to canvas filters, so it makes sense to have
Gfx::Filter that can be used by both libraries in an analogous way
as Gfx::Color.
