commit 1f6af5c4
"vcl: move Bitmap::MakeMonochrome() to BitmapMonochromeFilter"
seems to have dropped a line of code which means the filtering
doesn't actually have an effect.

Change-Id: If46c15bd9ab9d4e3911bdb9baf3091889b1f634d
Reviewed-on: https://gerrit.libreoffice.org/55071Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>