fdo#72400: slideshow: display a snapshot of movie until its not started

Change-Id: Ibd81f2aada458dd7437053b59882d3200f78747e
(cherry picked from commit 539017f1d96b0ca2c062ed7ffd2f48166f831493)
Reviewed-on: https://gerrit.libreoffice.org/8949
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>