when a window has 100% width or height (common in the panel),
it disables the shadows on the sides (or top/bottom) and
since they are used to compute width/height, if all 4 corner
shadows are disabled, it will think the resulting shadow pixmap
is null having width or height == 0
in this case, use the width of left and right pixmaps for the width
and height of top and bottom for the height, as they will be the only elements present