instead of manual reference counting.

Also the releasing of not-currently-in-use LogicalFontInstance objects
from the cache is made less aggressive - we now only flush entries until
we have less than CACHE_SIZE instances, instead of flushing the whole
cache.

Change-Id: Ib235b132776b5f09ae8ae93a933c2eebe5fa9610
Reviewed-on: https://gerrit.libreoffice.org/55384Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>