[css-borders-4] Add a way to turn off border image slicing

As noted in different places like #9183, #9714 or https://css-tricks.com/understanding-border-image/#comments, the 9-slice scaling algorithm behind border images is hard to grasp for authors.
Also, it doesn't cover use cases like adding an image that spans the whole border image area.

Therefore, an option should be added that allows to choose a different algorithm to render the border image across the whole border image area without slicing it.

One way could be to introduce a new property border-image-layout with the values slice and no-slice (names obviously to be bikeshedded).

no-slice causes border-image-slice to have no effect on the image and that the image defined in border-image-source is displayed within the border image area as a whole. border-image-repeat then tiles the whole image. The other border-image-* properties behave the same.

That covers the use cases mentioned in #8802 and #9456, in particular.

Sebastian

Start by reviewing the css-borders-4 context and the related issues #9183, #9714, #8802, and #9456. Clarify the proposed border-image-layout behavior, including its interaction with border-image-slice and border-image-repeat, then determine the specification changes needed for the agreed design.