1) after many trials and errors what LibO apparently expects here is
the ID of QWindow parent of [embedded] system child window

2) the position of the embedded window for video overlay has already
been translated relative to the top-left corner of the slide (see
bugfix of tdf#42873 how) in gen, gtk and kde5 vclplugs. So let's limit
translating it 2nd time only to gtk3 vclplug which for some reason
behaves differently 
(regression from 18138417)

Change-Id: Ie4c6d14a50959c0fdd04e745918d4889c9da45ec
Reviewed-on: https://gerrit.libreoffice.org/72458
Tested-by: Jenkins
Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>