[css-backgrounds-3][css-images-4] Change background-image animation from discrete to following default interpolation of <image> from CSS Images 4

[weinig]

The background-image property is currently specified as having "Animation type: discrete", which seems to indicate that it should follow the discrete rules defined in Web Animations.

At the same time, CSS Images 4 defines general interpolation for images that seems like a nicer behavior as you get things like cross-fades and nice gradient interpolation.

I propose updating background-image to allow using the default <image> interpolation from CSS Images 4.

[ydaniv]

+1!

Should probably also include the rest of the properties that take <image>, but then these may take other value types.
Some examples: mask-image, border-image, cursor, content.