Despite that by values should be offset values instead of
ending values, what we got from pptx seems to be the later,
that we have to subtract 100% from the by values to get
offset values for slideshow.

Change-Id: I7eb9981210ea9998dae0de8f22c155b394e5abeb
Reviewed-on: https://gerrit.libreoffice.org/57860
Tested-by: Jenkins
Reviewed-by: Mark Hung <marklh9@gmail.com>