wayland: Ensure the touch sequence pointer value is non-null

Weston numbers its touch sequences ids starting from 0, thus simply
setting the GtkEvents touch.sequence to the touch id value typically
causes gdk_event_get_event_sequence to return NULL. Unfortunately this
confuses other parts of GDK.

As both weston & mutter keep the sequence id between 0..max_dev_touches
-1 simply use + 1 to keep the id > 0. While this isn't entirely correct
(compositor could send -1 as the touch id), this keeps the touch id in
gtk tied to the touch id from weston which is useful for debugging. A
more thorough solution could be done when it turns out this is an issue
in practise

https://bugzilla.gnome.org/show_bug.cgi?id=731371

gdk/wayland/gdkdevice-wayland.c

@@ -1289,7 +1289,7 @@ _create_touch_event (GdkWaylandDeviceData *device,
   event->touch.time = time;
   event->touch.state = device->modifiers;
   gdk_event_set_screen (event, display->screen);
-  event->touch.sequence = GUINT_TO_POINTER (touch->id);
+  event->touch.sequence = GUINT_TO_POINTER (touch->id + 1);
 
   if (touch->initial_touch)
     {