SDL_SemPost -- Unlock a semaphore.
int SDL_SemPost(SDL_sem *sem);
SDL_SemPost unlocks the semaphore pointed to by
sem and atomically increments the semaphores value.
Threads that were blocking on the semaphore may be scheduled after this call
SDL_SemPost should be called after a semaphore is locked by a successful call to
Returns 0 if successful or
-1 if there was an error (leaving the semaphore unchanged).