The new callback time interval, or 0 to disable further runs of the callback.
Threadsafety: SDL may call this callback at any time from a background thread; the application is responsible for locking resources the callback touches that need to be protected.
Function prototype for the millisecond timer callback function.
The callback function is passed the current timer interval and returns the next timer interval, in milliseconds. If the returned value is the same as the one passed in, the periodic alarm continues, otherwise a new alarm is scheduled. If the callback returns 0, the periodic alarm is canceled and will be removed.