fdo#70838: Fix position issue when exporting shapes to docx.

Fixed an error at ImplEESdrWriter::ImplFlipBoundingBox implementation.
I've also tried to simplify and explain the calculations done there.

Change-Id: I41c6c6e1a045803524479c92ef807913db38167a
Reviewed-on: https://gerrit.libreoffice.org/6433
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>