Match CSS for background extents

The CSS specification says that the background extends to the
edge of the border (settable in CSS3 with border-clip), make
BigRectangle match this by computing an "effective border color"
as 'border OVER background'.

(If we don't want this behavior - e.g., to be able to use the
transparent borders as margins, then alternatively transparent
border handling would have to be fixed in st-widget.c, since
prior to this transparent and translucent borders were handled
differently.)

https://bugzilla.gnome.org/show_bug.cgi?id=595993