Don't update position of widgets in a wxPizza from size_allocate handler.
The values in m_{x,y} are not the same as client coordinates if the wxPizza is scrolled.
Closes #15444


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74733 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775