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