OOXML PresentationML doesn't support direction-relative paragraph
alignment. This change updates the Impress OOXML filter to flatten
start/end alignment into left/right, depending on writing direction.
Change-Id: I85b764cbd43b95646dcf00ba287f02216cd76b9b
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/197486
Tested-by: Jenkins
Reviewed-by: Jonathan Clark <jonathan@libreoffice.org>