Detach image data when returning an ImageFrame as a native image.

With error analysis and fixing guidance by Allan Sandfeld Jensen.

This caused crashes when a gif was being loaded and the data in the
backing store for image frame was prematurely freed.

Task-number: QTBUG-31936
Change-Id: I7f4454db00cbbcf53f18060aa986a843c7522d57
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>