Fixed broken lookup of fallback fonts on qpa

Fixed broken lookup of fallback fonts on qpa

Loading of fallback fonts from the font cache was broken.
While iterating through fallback fonts, we would incorrectly skip any
fonts which were already in the cache.  This would cause unstable lookup
results for some fonts, depending on the order in which fonts were
loaded during the current process.

Change-Id: I56a6a07e93196ed14f33f4cc181e41c5b8f19498
Reviewed-on: http://codereview.qt.nokia.com/3059
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Jiang Jiang <jiang.jiang@nokia.com>
Reviewed-by: Rohan McGovern <rohan.mcgovern@nokia.com>