gdk_window_get_back_pixmap() and gdk_window_get_background() have
been removed/deprecated. Use gdk_window_get_background_pattern()
as replacement.
https://bugzilla.gnome.org/show_bug.cgi?id=626583
In order to replace calls to deprecated GDK code, the current
development version of GTK+ is required. Add some basic compatibility
code to allow building mutter with GTK+ 2.18.
https://bugzilla.gnome.org/show_bug.cgi?id=587991