Fix crash when app font is added

Fix crash when app font is added

Loading app fonts will clear the application font cache, but
QFontPrivate::engineWithScript will try to load the font again,
in Mac the font engine used here must be the one used for shaping,
because subsequent sub font engines may be added to it during the
shaping process (QCoreTextFontEngineMulti::stringToCMap). That is
why we need to fetch the font engine directly from QTextEngine's
fontEngine cache instead of QFontCache.

Task-number: QTBUG-20250
Reviewed-by: Eskil
(cherry picked from commit 1f90ae36cff8acf581d1624bf011fe3a55c623c0)

Change-Id: Ibc0054cd7df65b65a67af4a7b15027731ba417fe
Reviewed-on: http://codereview.qt.nokia.com/1630
Reviewed-by: Jiang Jiang <jiang.jiang@nokia.com>
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>