The new timer model is formalized as using a separate thread to handle timer callbacks. This was the case on almost every platform before, but it's now a requirement, and simplifies the implementation and makes it perform consistently across platforms.
Goals:
* Minimize timer thread blocking
* Dispatch timers as accurately as possible
* SDL_AddTimer() and SDL_RemoveTimer() are completely threadsafe
* SDL_RemoveTimer() doesn't crash with a timer that's expired or removed
* SDL_timer_running wasn't always updated correctly.
* Fixed occasional crash in SDL_SetTimer() when clearing threaded timers
* It was possible to get both the timer thread and event thread running
* Other misc. cleanup
