Checks if the semaphore is a zombie (it can't be resolved anymore). This happens when one of the new chunks has been disposed before receiving its vertex data. If this is the case, we resolve the semaphore immediately
Updates the state of the semaphore
The DeleteSemaphore is responsible to delete chunk only when replacements are created to avoid holes in the surface of planets Each time a replacement chunk is set ready, we decrement the countdown. When it reaches 0 the old chunks can be deleted