this is motivated by the new screenshot feature
the initial proposed solution involved running make screenshot once per lang
which took ~6 hours for --with-lang=ALL on tb68 a reasonnably big
windows slavebot.

with this patch, one can run make screenshot just once and get all the screenshot
the elapsed time is 36 inutes on the same box/same config a 10x improvement.

Change-Id: I4339caebf915c118aa455de2a7e56e1a4e413939
Reviewed-on: https://gerrit.libreoffice.org/30970Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>