Keep web fontdata alive as long as CG uses it

With OS X 10.9 font decoding appears to happen lazily,
this means we have to ensure the data we provide CG is
kept alive until it dereferences it itself.

Task-number: QTBUG-34332
Change-Id: I8fc38fafba746b062c4ad16314b0a410fd0b668d
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>
(cherry picked from qtbase/89ab2671051671ecf440ba480caf0b6b6768b1a9)
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>