Avoid inadvertently destroying a shared PangoContext on GTK2

wxMemoryDC and wxScreenDC dtors were freeing a context they didn't own, potentially
causing a crash. Fixed by having the base wxWindowDC dtor free the context.
See #18566

src/gtk/dataview.cpp

@@ -2629,6 +2629,7 @@ public:
         m_window = window;
 
         m_context = window->GTKGetPangoDefaultContext();
+        g_object_ref(m_context);
         m_layout = pango_layout_new( m_context );
         m_fontdesc = pango_font_description_copy(gtk_widget_get_style(widget)->font_desc);