Reuse the same worker instead of creating a new one for each call

This reduces the average demo resize times by another 20-30ms on top of my other PR (#6) by reusing the same worker for all resizes rather than spawning a new one each time. This obviously assumes that you're only resizing one image at a time, but I'd say this is the common case.  Really there should be some kind of thread pool with one thread for each core or something.