Don't create the Cogl GLib source multiple times

Don't create the Cogl GLib source multiple times

Since the check for backend->cogl_context was accidentally moved
to clutter_backend_do_real_create_context, the Glib source that
is created at the end of clutter_backend_do_create_context() is
created and added each time create_context() is called, though
create_context() is supposed to be idempotent.

https://bugzilla.gnome.org/show_bug.cgi?id=768243

Gbp-Pq: Name Don-t-create-the-Cogl-GLib-source-multiple-times.patch