Force a low Z order of ShadowedRectangle's software fallback
PaintedRectangle, the item used as software rendering fallback, is an item that gets added to the normal list of children. This means it is part of normal Z order calculations, instead of being internal to the item. So we need to force it into a low Z order to ensure it always acts as "background"
cc @fvogt
Edited by Arjen Hiemstra