wayland: Dispatch pending events before entering poll

If we don't dispatch the pending events then we can enter poll with events
still requiring to be processed and which can then lead to us deadlocking
there.

gdk/wayland/gdkeventsource.c

@@ -50,6 +50,9 @@ gdk_event_source_prepare(GSource *base, gint *timeout)
     return TRUE;
 
   if (wl_display_flush (display->wl_display) < 0)
+    g_error ("Error flushing display: %s", g_strerror (errno));
+
+  if (wl_display_dispatch_pending (display->wl_display) < 0)
     g_error ("Error dispatching display: %s", g_strerror (errno));
 
   return FALSE;