border-image: add support for border-image-width too

border-image: add support for border-image-width too

It's useful to set a slice size != border-width, as backgrounds are
clipped to border-width too.

As slices can be half-transparent and overlap the background,
this would not fill the border box properly if we only use a single
property for specifying the width.

Also, this brings us even closer to CSS3.