so LibreOffice thinks its window is smaller than what gtk3 has allocated
for it.

For gtk3 (like firefox does) split size and position handling, leave
position/move handling to the configureEvent, but listen to sizeAllocate
and use that for the size handling.

Leave gtk2 as it always was

(cherry picked from commit 6adfbe62)

Change-Id: Ic52d6971595741ed658247b651e9e16c2ef9ed0b
Reviewed-on: https://gerrit.libreoffice.org/19924Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>