Thumbnail thread exit problem

If we're in thumbnail view, the thumbnail thread could be running, and we want it to exit, so we can safely launch another instance. We ask it to exit by incrementing the job ID; the thread checks the job ID after each extraction, and exits if the ID changes. This simple method has a weakness: there's a chance we preempted the thread after it checked the ID, but before it posted the bitmap to us, in which case we'll receive a spurious bitmap message. The thread is almost always busy extracting, so the chance is very small, but just in case, we increment the job ID before doing other work, thereby giving the thread more time to exit.