fdo#78427 PDF Import: Improve detection of bold italic font

(Bug #78427 is fixed only partially by this commit)

Change-Id: I080dca98a77a645c4c5ae19a9bbcae7d54179d3c
Reviewed-on: https://gerrit.libreoffice.org/9276
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>