Backport three patches from libepubgen master branch to support
exporting style:writing-mode as body CSS styles.

1. Support writing-mode for reflowable layout method.
https://sourceforge.net/p/libepubgen/code/ci/9a284081eea4a95235a6d6a6a50cbe3f7ad323ba/

2. Always keep page properties when splitting.
https://sourceforge.net/p/libepubgen/code/ci/0318031b9094b9180d1d391d0ca31a782b016e99/

3. Ensure page properties in the page span work
https://sourceforge.net/p/libepubgen/code/ci/1f602fcaa74fc9dbc6457019d11c602ff4040a4e/

Change-Id: I9033cb1f5fcbfedb423308fb29b9bd4d6d7d7a43
Reviewed-on: https://gerrit.libreoffice.org/52083Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>