title: vkAcquireNextImageKHR semantics? What does timing out mean?
body: <p>I am confused about the semantics of <code>vkAcquireNextImageKHR</code>.  Specifically, under what circumstances can the function return <code>VK_TIMEOUT</code>?  ie What could block it from not being able to acquire an image immediately?  What might it wait for to happen?</p>
  
  <p>It seems that an image can be acquired even if the presentation engine hasn't finished reading from the image, and one needs to use a semaphore or fence to synchronize usage of the acquired image anyway.  What am I missing?</p>