Fix regression from:

commit dc9ee533
Author: Tomaž Vajngerl <tomaz.vajngerl@collabora.co.uk>
Date:   Fri Jun 15 19:32:15 2018 +0200

    vcl: parser of font features included in the font name

Where hb_feature_from_string() was replaced by a simple parser that supports
avery limited subset of the syntax it supports (as documented in
https://harfbuzz.github.io/harfbuzz-hb-common.html#hb-feature-from-string)

Change-Id: I613190a677d24183e8c718fcfcaf9cf9b37a1e8f
Reviewed-on: https://gerrit.libreoffice.org/69062Reviewed-by: Khaled Hosny <khaledhosny@eglug.org>
Tested-by: Khaled Hosny <khaledhosny@eglug.org>