GtkPopover: Don't resize during the transition

Call gtk_popover_update_position instead which will pick up the new
transition_diff value and pass it on to
_gtk_window_set_popover_position, which in turn will move the window
correctly.

https://bugzilla.gnome.org/show_bug.cgi?id=755435

gtk/gtkpopover.c

@@ -517,7 +517,7 @@ show_animate_cb (GtkWidget     *widget,
       gtk_widget_set_opacity (widget, 1.0 - t);
     }
 
-  gtk_widget_queue_resize (GTK_WIDGET (popover));
+  gtk_popover_update_position (popover);
 
   if (t >= 1.0)
     {