Fix GetBestSize() for GTK3 after size has been set

Need to reset size request to get actual best size.
See Fedora bug https://bugzilla.redhat.com/show_bug.cgi?id=1282142

(cherry picked from commit 6ed7e27bf2)

src/gtk/control.cpp

@@ -319,7 +319,11 @@ wxSize wxControl::GTKGetPreferredSize(GtkWidget* widget) const
 {
     GtkRequisition req;
 #ifdef __WXGTK3__
+    int w, h;
+    gtk_widget_get_size_request(widget, &w, &h);
+    gtk_widget_set_size_request(widget, -1, -1);
     gtk_widget_get_preferred_size(widget, NULL, &req);
+    gtk_widget_set_size_request(widget, w, h);
 #else
     GTK_WIDGET_GET_CLASS(widget)->size_request(widget, &req);
 #endif