QSGRenderContext was holding pointers to QFontEngines without
increasing the font engine's ref count, allowing them to be deleted
before QSGRenderContext could make use of them.

Task-number: QTBUG-36573
Task-number: QTBUG-38313
Change-Id: I0ed28bb44882c55f330c27c23b533b7999d7e04b
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>