Avoids a rather horrible flickering problem in GL mode.

Squashing:

tdf#94213 - release offscreen texture properly on re-size.

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.

Squashing:

tdf#94213 - cleanup associated GL contexts properly when DCs released.

Change-Id: I6b9fb899777d8e460999ac3ff038a1302e434bb5
Reviewed-on: https://gerrit.libreoffice.org/18607Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tor Lillqvist <tml@collabora.com>