Fix ClientToScreen()/ScreenToClient() when used immediately after window creation.

And whenever window does not have an up-to-date GTK size allocation.
Closes #16061


git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78033 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/gtk/window.h

@@ -255,6 +255,8 @@ public:
     // (because they have been validated by a size-allocate) and should
     // be used to report client size
     bool m_useCachedClientSize;
+    // Whether the GtkAllocation and GdkWindow positions are known to be correct
+    bool m_isGtkPositionValid;
 
     // see the docs in src/gtk/window.cpp
     GtkWidget           *m_widget;          // mostly the widget seen by the rest of GTK