First frame always has lower score than expected

[soCzech]

As mentioned in issue #1159, the first frame of a video has lower than expected VMAF score. This is the case even if both videos are the same, e.g., vmaf_resource/python/test/resource/yuv/src01_hrc00_576x324.yuv.

In detail, running:

./libvmaf/build/tools/vmaf
    --reference ./vmaf_resource/python/test/resource/yuv/src01_hrc00_576x324.yuv
    --distorted ./vmaf_resource/python/test/resource/yuv/src01_hrc00_576x324.yuv
    --width 576 --height 324 --pixel_format 420 --bitdepth 8 --json -o test.json --feature psnr=enable_mse=true

yields this output:

{
  "version": "b9ac69e6",
  "fps": 85.26,
  "frames": [
    {
      "frameNum": 0,
      "metrics": {
        "integer_adm2": 1.000002,
        "integer_adm_scale0": 1.000015,
        "integer_adm_scale1": 1.0000003037306016,
        "integer_adm_scale2": 1.0000006262280157,
        "integer_adm_scale3": 1.0000005666229612,
        "integer_motion2": 0.000000,
        "integer_motion": 0.000000,
        "integer_vif_scale0": 1.000000,
        "integer_vif_scale1": 1.000000,
        "integer_vif_scale2": 0.999999,
        "integer_vif_scale3": 0.999999,
        "psnr_y": 60.000000,
        "mse_y": 0.000000,
        "psnr_cb": 60.000000,
        "mse_cb": 0.000000,
        "psnr_cr": 60.000000,
        "mse_cr": 0.000000,
        "vmaf": 97.428478
      }
    },
    {
      "frameNum": 1,
      "metrics": {
        "integer_adm2": 1.000002,
        "integer_adm_scale0": 1.000015,
        "integer_adm_scale1": 1.0000003102343520,
        "integer_adm_scale2": 1.0000006381999698,
        "integer_adm_scale3": 1.0000005540160577,
        "integer_motion2": 4.215680,
        "integer_motion": 4.580830,
        "integer_vif_scale0": 1.000000,
        "integer_vif_scale1": 1.000000,
        "integer_vif_scale2": 0.999999,
        "integer_vif_scale3": 0.999999,
        "psnr_y": 60.000000,
        "mse_y": 0.000000,
        "psnr_cb": 60.000000,
        "mse_cb": 0.000000,
        "psnr_cr": 60.000000,
        "mse_cr": 0.000000,
        "vmaf": 100.000000
      }
    },
    ...
  ],
  ...
}

All frames of the video have VMAF 100.0 except for the first one.

To verify this behaviour, I prepended a black frame at the beginning of the video -- then the first black frame has VMAF score of 97.428043 and the actual first frame (now as the second frame) has score 100.0.