x11: Introduce a meta-x11-types header

Used to define x11 only types. Future commits will introduce
a way for compositors to detect if they can use x11 features or not

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3776>

src/tests/meta-context-test.c

@@ -26,6 +26,7 @@
 #include <gio/gsettingsbackend.h>
 
 #include "core/meta-context-private.h"
+#include "meta/meta-x11-display.h"
 #include "tests/meta-backend-test.h"
 #include "tests/meta-test-shell.h"
 #include "tests/meta-test-utils-private.h"

src/tests/meta-test-utils.h

@@ -25,6 +25,7 @@
 
 #include "backends/meta-backend-types.h"
 #include "backends/meta-virtual-monitor.h"
+#include "meta/meta-x11-types.h"
 #include "meta/window.h"
 
 #define META_TEST_CLIENT_ERROR meta_test_client_error_quark ()