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/backends/meta-backend-private.h

@@ -209,3 +209,9 @@ void meta_backend_update_from_event (MetaBackend  *backend,
 
 char * meta_backend_get_vendor_name (MetaBackend *backend,
                                      const char  *pnp_id);
+
+META_EXPORT_TEST
+uint32_t meta_clutter_button_to_evdev (uint32_t clutter_button);
+
+META_EXPORT_TEST
+uint32_t meta_evdev_button_to_clutter (uint32_t evdev_button);