Qt change f5c5e20ab20f016c0735 optimizes the calculation of the
children's bounding rect by taking the parent bounding rect into
account. This led to a recursion in the QuickDesigner Form Editor,
because LayerItem::boundingRect() is defined as the children's bounding
rect.

Break the cycle by setting ItemClipsChildrenToShape to false.
(cherry picked from commit 2067cfcf)