seat-impl: Dispatch source also when there are already queued events

Libinput will queue a few initial events when a seat is assigned to the
udev backend; a result of it probing udev adding detected devices. For
us to see these events, we need to dispatch libinput before going idle,
as nothing will show up on the libinput file descriptor until something
else (e.g. keyboard event or mouse movement) wakes us up.

Do this by adding a prepare() function to the libinput GSource, that
checks whether there are any events in the queue already, and return
TRUE if so is the case, causing us to dispatch before going fully idle.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1688>

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

@@ -943,6 +943,25 @@ meta_seat_impl_notify_touch_event_in_impl (MetaSeatImpl       *seat_impl,
 /*
  * MetaEventSource for reading input devices
  */
+
+static gboolean
+meta_event_prepare (GSource *g_source,
+                    int     *timeout_ms)
+{
+  MetaEventSource *source = (MetaEventSource *) g_source;
+  MetaSeatImpl *seat_impl = source->seat_impl;
+
+  *timeout_ms = -1;
+
+  switch (libinput_next_event_type (seat_impl->libinput))
+    {
+    case LIBINPUT_EVENT_NONE:
+      return FALSE;
+    default:
+      return TRUE;
+    }
+}
+
 static gboolean
 meta_event_check (GSource *source)
 {
@@ -1429,7 +1448,7 @@ meta_event_dispatch (GSource     *g_source,
 }
 
 static GSourceFuncs event_funcs = {
-  NULL,
+  meta_event_prepare,
   meta_event_check,
   meta_event_dispatch,
   NULL