Replace deprecated GDK symbols

The fix depends on new API added in this cycle, so the required
GTK+ version is bumped to 2.21.1.

https://bugzilla.gnome.org/show_bug.cgi?id=587991

src/ui/ui.c

@@ -475,7 +475,7 @@ get_cmap (GdkPixmap *pixmap)
 
   /* Be sure we aren't going to blow up due to visual mismatch */
   if (cmap &&
-      (gdk_colormap_get_visual (cmap)->depth !=
+      (gdk_visual_get_depth (gdk_colormap_get_visual (cmap)) !=
        gdk_drawable_get_depth (pixmap)))
     {
       cmap = NULL;