backend: Introduce helpers for turning from/to clutter/evdev buttons

We have multiple implementations of these types of transformations, and
one is broken. Instead of just fixing it, add this helper so that it can
use it instead.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3325>

src/tests/button-transform-tests.h

@@ -0,0 +1,21 @@
+/*
+ * Copyright (C) 2023 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/>.
+ *
+ */
+
+#pragma once
+
+void init_button_transform_tests (void);