Create a new mutex.
Returns the initialized and unlocked mutex or NULL on failure; call SDL_GetError() for more information.
All newly-created mutexes begin in the _unlocked_ state.
Calls to SDL_LockMutex() will not return while the mutex is locked by another thread. See SDL_TryLockMutex() to attempt to lock without blocking.
SDL mutexes are reentrant.
This function is available since SDL 2.0.0.