QQuickTextEdit: render inline images after change 48798

The refactoring in QQuickTextEdit from this change:
https://codereview.qt-project.org/#change,48798


accidentally broke rendering of inline images.
Revive the ProtectedLayoutAccessor trick to get the proper
format and properly extract the image from it.

Change-Id: I445914efcea1012daa79d594aa1e828b1039b988
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>