Indefinite duration isn't necessary for indefinite repetition,
so simply check the number of iteration left and do not decrease
the number when the repetition is indefinite.

Note: a known issue is that the next effect starts before
"until click" or "until next slide" if the starting time of
the next slide is "after previous". A workaround is to set
the starting time of the next slide to "click".

Change-Id: Ie9c73b472dc400bc41501029ff19ecc3e3fab2df
Reviewed-on: https://gerrit.libreoffice.org/67896
Tested-by: Jenkins
Reviewed-by: Mark Hung <marklh9@gmail.com>