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 nbtk-widget.c, since
prior to this transparent and translucent borders were handled
differently.)
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