backends/native: Make some MetaSeatImpl API "async"

This API is the one accessed from different bits of the UI thread,
make it "async" (it's basically one-way setters, so API stays the same
in the surface) and able to run in the MetaSeatImpl main context.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1403>

src/backends/native/meta-seat-impl.h

@@ -54,6 +54,7 @@ struct _MetaSeatImpl
 {
   GObject parent_instance;
 
+  GMainContext *input_context;
   MetaSeatNative *seat_native;
   char *seat_id;
   MetaEventSource *event_source;
@@ -116,6 +117,10 @@ G_DECLARE_FINAL_TYPE (MetaSeatImpl, meta_seat_impl,
 MetaSeatImpl * meta_seat_impl_new (MetaSeatNative *seat_native,
                                    const char     *seat_id);
 
+void meta_seat_impl_run_input_task (MetaSeatImpl *seat_impl,
+                                    GTask        *task,
+                                    GSourceFunc   dispatch_func);
+
 void meta_seat_impl_notify_key (MetaSeatImpl       *seat_impl,
                                 ClutterInputDevice *device,
                                 uint64_t            time_us,