Fix bidi reordering when part of text is rendered by fallback font

Fix bidi reordering when part of text is rendered by fallback font

If the fallback font is used for part of a RTL text, we need to
position the different text items accordingly, subtracting the advance
instead of adding it.

Task-number: QTBUG-17117
Done-with: Lars
(cherry picked from commit e5e1ff0d6f4e6a8457da61b5b215730de6f960bd)

Change-Id: I73b501d36e5c9e12112cc1997c1d360f3a4a6083
Reviewed-on: http://codereview.qt.nokia.com/1228
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@nokia.com>