Using vdev's associated with old contexts, is un-necessary and a
recipe for context switching & hence flicker.

Change-Id: I37ed967a7816e5ca0240908ab4793ce1e68570ee
Reviewed-on: https://gerrit.libreoffice.org/18602Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Reviewed-on: https://gerrit.libreoffice.org/18608Reviewed-by: Tor Lillqvist <tml@collabora.com>