MetaBackend: Move fields from MetaBackend into MetaBackendPrivate

Lets prepare for being able to declare the MetaBackend type using
the macro helper.

https://bugzilla.gnome.org/show_bug.cgi?id=777732

src/backends/native/meta-backend-native.c

@@ -154,7 +154,8 @@ constrain_to_client_constraint (ClutterInputDevice *device,
                                 float              *y)
 {
   MetaBackend *backend = meta_get_backend ();
-  MetaPointerConstraint *constraint = backend->client_pointer_constraint;
+  MetaPointerConstraint *constraint =
+    meta_backend_get_client_pointer_constraint (backend);
 
   if (!constraint)
     return;