Keep Multi font engine in QFontCache

Keep Multi font engine in QFontCache

If we only keep single QFontEngineFT in QFontCache, then when we
are looking for the MultiQPA next time with the same QFontDef,
the FT engine will be returned and we get no font fallback support.
That's why we need to keep the Multi engines in QFontCache as well
and distinguish them from the single item. Since QPA doesn't use
'screen' field of QFontCache::Key structure, we use it here to
indicate that we are caching a multi font engine.

Change-Id: Id899d5c5ba52f4bccf134bcd6b1c6386ba22063c
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@nokia.com>