Make mutter manage its own clutter backends

Introduce two new clutter backends: MetaClutterBackendX11 and
MetaClutterBackendNative. They are so far only wrap ClutterBackendX11
and ClutterBackendEglNative respectively, but the aim is to move things
from the original clutter backends when needed.

https://bugzilla.gnome.org/show_bug.cgi?id=768976

src/backends/native/meta-clutter-backend-native.h

@@ -0,0 +1,38 @@
+/* -*- mode: C; c-file-style: "gnu"; indent-tabs-mode: nil; -*- */
+
+/*
+ * Copyright (C) 2016 Red Hat
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA
+ * 02111-1307, USA.
+ *
+ * Written by:
+ *     Jonas Ådahl <jadahl@gmail.com>
+ */
+
+#ifndef META_CLUTTER_BACKEND_NATIVE_H
+#define META_CLUTTER_BACKEND_NATIVE_H
+
+#include <glib-object.h>
+
+#include "clutter/clutter.h"
+#include "clutter/egl/clutter-backend-eglnative.h"
+
+#define META_TYPE_CLUTTER_BACKEND_NATIVE (meta_clutter_backend_native_get_type ())
+G_DECLARE_FINAL_TYPE (MetaClutterBackendNative, meta_clutter_backend_native,
+                      META, CLUTTER_BACKEND_NATIVE,
+                      ClutterBackendEglNative)
+
+#endif /* META_CLUTTER_BACKEND_NATIVE_H */