It's cleaner than streaming the font and then hashing the result,
and it's also faster.
Change-Id: I6262e45362d386c21482f1e71be51912f123ee45
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/133500
Tested-by: Jenkins
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>
