fdo#74468 fix timing of slide transitions

Regression since commit 16428c9600964a4945cf6fd0d938dea047d1248b.

Change-Id: Id274c21e08d10d2e727f3b5a3fd852cd297e4637
(cherry picked from commit 78ff9d90f2a74792c437087ede11559c111c5c98)
Reviewed-on: https://gerrit.libreoffice.org/8029
Reviewed-by: Fridrich Strba <fridrich@documentfoundation.org>
Tested-by: Fridrich Strba <fridrich@documentfoundation.org>