title: Use of waitUntilCompleted
body: <p>When I run a CommandBuffer with several passes, I normally attach to the end a <code>waitUntilCompleted</code> command. This time though, even if there are several encoders/command buffers, I present the data directly to the screen, so I don't need to pass anything to the CPU. I guess I can just do without the wait, is that right?</p>
  
  <p>What about a test though? What would be the best way to wait for the GPU to finish from outside a class?</p>