status icon: Prevent size dithering

We're seeing loops where the size of some status icons constantly
dithers between 24 and 25. Since I couldn't track down exactly
where the one extra pixel comes from, just stop reacting
to single-pixel size changes.

https://bugzilla.gnome.org/show_bug.cgi?id=758893

gtk/gtkstatusicon.c

@@ -1690,7 +1690,7 @@ gtk_status_icon_size_allocate (GtkStatusIcon *status_icon,
   priv->image_width = allocation->width - GTK_MISC (priv->image)->xpad * 2;
   priv->image_height = allocation->height - GTK_MISC (priv->image)->ypad * 2;
 
-  if (priv->size != size)
+  if (priv->size - 1 > size || priv->size + 1 < size)
     {
       priv->size = size;