We used to render them in the app main (GUI) thread, which is
dangerous, accessing the headless frames in one thread while the LO
thread (where the "main" LibreOffice code is running) might be
updating and changing them.

This fixes some problems like that part of the document did not show
up. If I would test the app on a multi-core iPad, presumably I would
have seen even more problems.

But this also introduces new problems: Now the UI doesn't appear
incrementally (for instance, with an actually progressing progress bar
during the loading of the document) as it used to. Now it all appeads
all of a sudden, everything at once. Which would be fine if it
happened very quickly after starting the app, but it doesn't... on the
original iPad it takes half a minute.

Change-Id: I04068e0d884aa5cb86acefa76449aac4e081b193