We used to special handle gradient, but we want to be able to store
other sources (that will eventually resolve to cairo patterns).
For instance, this is needed to handle win32 theme part sources.
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.
