backend: Move GPU ownership from the monitor manager to the backend

Lets work towards making MetaMonitorManager about managing monitors, and
not about managing GPUs. This changes other units to keep a pointer to
the backend instead of a monitor manager, in case their ownership
changed, or their main usage of the monitor manager was to look up GPUs.

https://gitlab.gnome.org/GNOME/mutter/issues/548
https://gitlab.gnome.org/GNOME/mutter/merge_requests/525

src/tests/meta-gpu-test.h

@@ -0,0 +1,26 @@
+/*
+ * Copyright (C) 2016-2018 Red Hat, Inc.
+ *
+ * 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, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef META_GPU_TEST_H
+#define META_GPU_TEST_H
+
+#include "backends/meta-gpu.h"
+
+#define META_TYPE_GPU_TEST (meta_gpu_test_get_type ())
+G_DECLARE_FINAL_TYPE (MetaGpuTest, meta_gpu_test, META, GPU_TEST, MetaGpu)
+
+#endif /* META_GPU_TEST_H */