I'm not sure this is the best idea, but without a context many
things will definitly break. In theory this is just needed for
bitmap-only rendering mode.

This is a more general solution of commit e659c6a1 ("Work
around odd macOS 10.14 graphicsContextWithWindow failure").

Change-Id: I3708c25b302a38e1b0c0cccbcacd6cb9b5ff33dc
Reviewed-on: https://gerrit.libreoffice.org/64952
Tested-by: Jenkins
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>