macos: Fix freeze on drag

A Drag surface does not have a parent surface. Therefore, if we initialize
it with one, it's inheriting the frame clock from the parent, but the
drag surface is not linked to the parent. Once the drag surface is destroyed,
it's disposing the frame clock, which results in a "frozen" application.

gdk/macos/gdkmacossurface.c

@@ -431,7 +431,7 @@ gdk_macos_surface_drag_begin (GdkSurface         *surface,
   _gdk_macos_surface_get_root_coords (GDK_MACOS_SURFACE (surface), &sx, &sy);
   drag_surface = _gdk_macos_surface_new (GDK_MACOS_DISPLAY (surface->display),
                                          GDK_SURFACE_DRAG,
-                                         surface,
+                                         NULL,
                                          sx, sy, 1, 1);
   drag = g_object_new (GDK_TYPE_MACOS_DRAG,
                        "drag-surface", drag_surface,