Be explicit about the page number, this way a video on the second page
doesn't end up as an annotation for the first page. (In the Impress case
each slide is exported separately, so there this wasn't a problem.)

Change-Id: I83ba9cb4a3b2a6734bd88a138654e391199651c6
Reviewed-on: https://gerrit.libreoffice.org/32696Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>