xwayland: Use CurrentTime on all XConvertSelection calls

The call fetching the targets mistakenly used the timestamp meant
to back up the TIMESTAMP atom (hence, it's the timestamp at which
the selection is *owned* by the compositor, on behalf of a wayland
client).

This timestamp is actually only updated when the compositor gets
to own the selection, so it's a randomly late timestamp to retrieve
the TARGETS atom content, which certain clients might end up
ignoring.

https://bugzilla.gnome.org/show_bug.cgi?id=768007

src/wayland/meta-xwayland-selection.c

@@ -1577,7 +1577,7 @@ meta_xwayland_selection_handle_xfixes_selection_notify (MetaWaylandCompositor *c
                              gdk_x11_get_xatom_by_name ("TARGETS"),
                              gdk_x11_get_xatom_by_name ("_META_SELECTION"),
                              selection->window,
-                             selection->timestamp);
+                             CurrentTime);
           XFlush (xdisplay);
         }
     }