The compositor IPC boundary needs to transfer CSS corner radii without
keeping their wire shape in Web-specific glue. Declare the IPC
specializations on the LibGfx types and encode each corner radius pair
next to the CornerRadii implementation.
This is preparatory work required to add IPC between the main and
compositor threads.
Prep for moving display-list rasterization out of LibWeb by moving the
device-pixel corner radius, corner clip, and anti-aliasing types to
LibGfx.
Keep BorderRadiiData in LibWeb as the CSS-pixel representation and
convert it to Gfx corner radii when recording display-list commands.