• Use ffmpeg -fps_mode passthrough for all sample encodes. This improves VMAF scores in some cases.
• Use explicit pixel format in xpsnr calls, use highest quality format of the ref & distorted streams.
  This can improve scores in some cases.
• Wait for vmaf/xpsnr processes to finish e.g. to allow custom logging to flush.
• Update sample-encode json output to use float predicted_encode_seconds.