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 succeeds.
SDL_SemPost should be called after a semaphore is locked by a successful call to
SDL_SemWait, SDL_SemTryWait or SDL_SemWaitTimeout.
Returns 0 if successful or -1 if there was an error (leaving the semaphore
SDL_CreateSemaphore, SDL_DestroySemaphore, SDL_SemWait, SDL_SemTryWait,