Added thread pool support to BitmapManager; WorkerThread is now pool-enabled; added BitmapManager::setNumThreads().

git-svn-id: https://www.libavg.de/svn/trunk/libavg@9121 44470bb9-56e9-0310-a0f8-c586564d3dc6
Added documentation for BitmapManager::setNumThreads().

git-svn-id: https://www.libavg.de/svn/trunk/libavg@9122 44470bb9-56e9-0310-a0f8-c586564d3dc6