Multithreaded tools

Concurrency would increase the performance of our project. Paint Tool SAI, from what I've observed, uses multi-threading to improve throughput of large brushes - most seen in brush sized 500, usage on my both cores ramps up to the maximum. Most tools should be susceptible to this treatment.
Usage of QtConcurrent would provide care-free implementation base.
We must test the overhead, though. For small brush sizes it might prove too expensive to launch this module.
One day perhaps a CUDA could be employed to fulfill this task.
Recent GSoC updates to krita introduced "multi-threaded canvas updating"; see http://krita.org/component/content/article/10-news/57-last-week-in-krita-week-32-and-a-bit-of-33. Since our code is somewhat simillar to theirs, we could peep how it is done.