update size hints when decoration size becomes known, and preserve size hint increments

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71856 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk/toplevel.h

@@ -150,6 +150,9 @@ protected:
 private:
     wxSize& GetCachedDecorSize();
 
+    // size hint increments
+    int m_incWidth, m_incHeight;
+
     // is the frame currently iconized?
     bool m_isIconized;