clutter: Explicitly initiate CoglMatrices

Instead of relying on the macro. The macro will go away in the
next commit as part of using graphene_matrix_t in the CoglMatrix
structure.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1439

clutter/clutter/clutter-scroll-actor.c

@@ -99,9 +99,11 @@ clutter_scroll_actor_set_scroll_to_internal (ClutterScrollActor     *self,
 {
   ClutterScrollActorPrivate *priv = self->priv;
   ClutterActor *actor = CLUTTER_ACTOR (self);
-  CoglMatrix m = COGL_MATRIX_INIT_IDENTITY;
+  CoglMatrix m;
   float dx, dy;
 
+  cogl_matrix_init_identity (&m);
+
   if (graphene_point_equal (&priv->scroll_to, point))
     return;