For tiled rendering this has the effect of not invalidating the
previous slide when switching slides. After switching slides, an
invalidation occurs caused by showing the new slide, and calling
'GetPart' before 'SwitchPage' finishes returned the old part number.

Change-Id: I1cafd0e51cd39be3a80d0559ae3051238b8df744
Reviewed-on: https://gerrit.libreoffice.org/17562Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>
Tested-by: Samuel Mehrbrodt <Samuel.Mehrbrodt@cib.de>