Add real-time mode to dav1d CLI

For power and CPU-usage testing a real-time mode would be extremely useful. In the CLI a --realtime parameter could be implemented, meaning if the .ivf states the video is 60fps dav1d decodes it at 60fps.

For more experimentation a --fps parameter could also be added which decodes a file at a certain fps.

And if frames are dropped, maybe dav1d can report the number of frames that weren't decoded in time.