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
BUG: 380825