This fixes the black rectangle instead of actual image content in the
bugdoc.

An alternative would be to handle this in the PNG import, but commit
66dbd4da (tdf#113918: Workaround: Load
1bpp indexed PNG as 8bpp indexed Bitmap, 2017-11-20) already tried
something similar and failed, leading to the revert in commit
25cd8436 (tdf#115297: alternative fix
for displaying 1bit images, 2018-02-27).

The test is especially useful with SAL_FORCEGL=1 SAL_USE_VCLPLUGIN=gen
specified on Linux, so the GL codepath is triggered.

Change-Id: Ia9d049d09dce2ac34826ee427f74616a96a35c88
Reviewed-on: https://gerrit.libreoffice.org/68925Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Tested-by: Jenkins