iconcache: don't crash on a non-alpha mask pixmap

We assume when applying a mask that it is a depth-1 pixmap that
cairo interprets as alpha. So ignore a non-depth-1 mask.

https://bugzilla.gnome.org/show_bug.cgi?id=641975

src/core/iconcache.c

@@ -434,7 +434,8 @@ try_pixmap_and_mask (MetaDisplay *display,
 
   if (unscaled && src_mask != None)
     {
-      get_pixmap_geometry (display, src_mask, &w, &h, NULL);
+      get_pixmap_geometry (display, src_mask, &w, &h, &d);
+      if (d == 1)
         mask = meta_gdk_pixbuf_get_from_pixmap (src_mask,
                                                 0, 0,
                                                 w, h);