main: Move MetaX11DisplayPolicy to a new meta-private-enums.h

As with the compositor type enum, also have the X11 display policy enum,
as it's also effectively part of the context configuration. But as with
the compositor type, move it to a header file for enums only, and since
this is a private one, create a private variant meta-enums.h.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1861>

src/core/main-private.h

@@ -21,13 +21,7 @@
 #define META_MAIN_PRIVATE_H
 
 #include "core/util-private.h"
-
-typedef enum _MetaX11DisplayPolicy
-{
-  META_X11_DISPLAY_POLICY_MANDATORY,
-  META_X11_DISPLAY_POLICY_ON_DEMAND,
-  META_X11_DISPLAY_POLICY_DISABLED,
-} MetaX11DisplayPolicy;
+#include "core/meta-private-enums.h"
 
 META_EXPORT_TEST
 MetaX11DisplayPolicy meta_get_x11_display_policy (void);

src/core/meta-private-enums.h

@@ -0,0 +1,31 @@
+/*
+ * Copyright (C) 2019-2021 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, write to the Free Software
+ * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA
+ * 02111-1307, USA.
+ *
+ */
+
+#ifndef META_PRIVATE_ENUMS_H
+#define META_PRIVATE_ENUMS_H
+
+typedef enum _MetaX11DisplayPolicy
+{
+  META_X11_DISPLAY_POLICY_MANDATORY,
+  META_X11_DISPLAY_POLICY_ON_DEMAND,
+  META_X11_DISPLAY_POLICY_DISABLED,
+} MetaX11DisplayPolicy;
+
+#endif /* META_PRIVATE_ENUMS_H */