Fix hb_face leaking in QFontEngineFT

Since HarfBuzz-old's HB_Face doesn't support ref-counting,
it is impossible to keep the same behavior as for NG's ref-counted hb_face
when we're going to reparent the data of unknown type in QFontEngineFT.

We should either not release the object returned by harfbuzzFace(),
or introduce ref-counting for HB-old's HB_Face. Stop referencing HB-NG's
objects on access for now and thus avoid a need to release them manually.

Task-number: QTBUG-42674
Change-Id: Ia21e7ba9c17185796b0dd98c2c27d02566f2a701
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@theqtcompany.com>