Unfortunately, since we can have 2x SalGraphics' on a OutputDevice,
and one of these can be a printer - things got very confused around
which context to glFlush. This de-tangles the various reference-counts.

Change-Id: I1062be0b02a91fc9009deaa3ec29c5dbb227df20
Reviewed-on: https://gerrit.libreoffice.org/18604Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Michael Meeks <michael.meeks@collabora.com>