Use deferred window positioning in wxWindow::Layout() in wxMSW.

Ensure that {Begin,End}RepositioningChildren() are called before/after
changing the positions of multiple children, whether we're using sizers or
constraints.

This fixes display corruption which could happen under Windows 7 and a child
resized first became bigger, pushing off the children resized later.

Closes #15214.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74067 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/sizer.cpp

@@ -976,6 +976,8 @@ void wxSizer::Layout()
     CalcMin();
 
     // Applies the layout and repositions/resizes the items
+    wxWindow::ChildrenRepositioningGuard repositionGuard(m_containingWindow);
+
     RecalcSizes();
 }