ZipPackageStream::saveChild() already had one threaded compression,
but that still uses only one thread for one stream. Many documents
contain many streams (where this is useful), but large documents
often contain one huge content.xml, which then would be compressed
using just one thread.
But it is in fact possible to do deflate in parallel on the same data,
at the cost of somewhat increased CPU usage (spread over threads).
This is handled separately from the background thread path, as
integrating these two approaches would probably be needlessly complex
(since they both internally use ThreadPool, the tasks should often
intermix and parallelize anyway).
On my 4-core (8 HT threads) machine this reduces the compression time
of tdf#113042 from 3s to 1s.
Reviewed-by: Luboš Luňák <firstname.lastname@example.org>