We need to ensure that we use an initialized context, and that
(when we re-parent) we DeInit and so reset the previous OpenGLContext.
Make UseContext more paranoid as well for good measure.

Change-Id: Ia45334222045e5d2f48da47560fab8511223a9a5
Reviewed-on: https://gerrit.libreoffice.org/18601Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>