[Backport to 3.38] clutter/backend: Don't dispatch libinput events too early

At startup, libinput dispatch is called from the MetaSeatNative constructed callback.

That means that we may get libinput events even before the default seat is set.

In turn, processing those events may trigger the use the default seat while it's still not set yet, and cause a crash of gnome-shell/mutter at startup.

A simple reproducer for this is to start gnome-shell/mutter with a tablet connected and the stylus in proximity, the proximity event will cause gnome-shell/mutter to crash at startup.

To avoid that issue, avoid dispatching libinput events early from the MetaSeatNative constructed callback, those events will eventually get processed when the seat and the backend are all setup.