wlc: gtk3 applications crash when trying to open a GtkMenu

Here. With above, GTK menus are shown without crashing and compositor can display them correctly positioned.

@Drakulix Will get on this bit later today. I need to remind myself on the positioner details before I can answer well.

@Cloudef Because this issue is really annoying me in my daily use, I would like to try to tackle it.

I browsed through the source code and got some questions:

• How would you like to see the positioner getting exposed by the api? It seems like it should almost be a new handle type. Alternatively don’t expose it and just respect it’s settings in the set_geometry call or stub it so it does not crash anymore.
• Would the positioner be a new resource in src/resources/types? How would I allocate it in the create_positioner call? Looking at the rest of the code, I just use wlc_resource_create and wlc_resource_implement, right? Implementing it then seems pretty straight-forward.
• Anything I am missing?

Views already have parent / child relationship, do the popups really need own?

Nope, I was not aware of that.

//edit: Anyway, that only means that last commit is not needed, just using view_get_parent works. I can tidy rest up and make PR.

Here’s the gdk_event_soure_prepare function from my local version of the source, with the line of the error message marked:

  static gboolean
  gdk_event_source_prepare (GSource *base,
                            gint    *timeout)
  {
    GdkWaylandEventSource *source = (GdkWaylandEventSource *) base;
    GdkWaylandDisplay *display = (GdkWaylandDisplay *) source->display;

    *timeout = -1;

    if (source->display->event_pause_count > 0)
      return _gdk_event_queue_find_first (source->display) != NULL;

    /* We have to add/remove the GPollFD if we want to update our
     * poll event mask dynamically.  Instead, let's just flush all
     * write on idle instead, which is what this amounts to.
     */

    if (_gdk_event_queue_find_first (source->display) != NULL)
      return TRUE;

    /* wl_display_prepare_read() needs to be balanced with either
     * wl_display_read_events() or wl_display_cancel_read()
     * (in gdk_event_source_check() */
    if (source->reading)
      return FALSE;

    /* if prepare_read() returns non-zero, there are events to be dispatched */
    if (wl_display_prepare_read (display->wl_display) != 0)
      return TRUE;
    source->reading = TRUE;

    if (wl_display_flush (display->wl_display) < 0)
*     g_error ("Error flushing display: %s", g_strerror (errno));

    return FALSE;
  }