Have video fit to the media object size on the slide,
allow more complicated interactions.

---

This also backports the relevant part from commit
bbe1ede0
("tdf#124027: use ID of the embedded window and fix
 position of  overlay") namely to make translation
gtk3-specific, to fix a regression, as mentioned in
its commmit message:

> 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)

Reviewed-on: https://gerrit.libreoffice.org/67978
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>
(cherry picked from commit 18138417)

Change-Id: Ice1fa4b521176ad7ed7f7d1d2b13e617e8282390
Reviewed-on: https://gerrit.libreoffice.org/73199
Tested-by: Jenkins
Reviewed-by: Mark Hung <marklh9@gmail.com>