Use client size determined by size-allocate when possible

Our client size calculation can be wrong for reasons beyond our control,
notably on Ubuntu 12.04 with overlay scrollbars where the GtkScrolledWindow
"scrollbar-spacing" is wrong. By using the true size from size-allocate
when it becomes available, we can recover from an earlier incorrect client
size calculation. See #14871


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73231 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/gtk/minifram.cpp

@@ -435,6 +435,10 @@ bool wxMiniFrame::Create( wxWindow *parent, wxWindowID id, const wxString &title
 void wxMiniFrame::DoGetClientSize(int* width, int* height) const
 {
     wxFrame::DoGetClientSize(width, height);
+
+    if (m_useCachedClientSize)
+        return;
+
     if (width)
     {
         *width -= 2 * m_miniEdge;