Flushing doesn't compute metrics for last frame when frames are submitted out of order

If I submit frames out of order, e.g. the last few frames are

3970
3974
3972
3973

When I try to compute a pooled vmaf_neg score for the entire sequence (after flushing), the function fails because frame 3974 does not have metrics computed for it. Looking at the output JSON, the last frame looks like this:

    {
      "frameNum": 3974,
      "metrics": {
        "integer_motion": 0.790710,
        "integer_adm2_egl_1": 0.974544,
        "integer_adm_scale0_egl_1": 0.966092,
        "integer_adm_scale1_egl_1": 0.960806,
        "integer_adm_scale2_egl_1": 0.970672,
        "integer_adm_scale3_egl_1": 0.983781,
        "integer_vif_scale0_egl_1": 0.707988,
        "integer_vif_scale1_egl_1": 0.951758,
        "integer_vif_scale2_egl_1": 0.970953,
        "integer_vif_scale3_egl_1": 0.978867
      }
    }

Notably it's missing the integer_motion2 metric.

My understanding is that after https://github.com/Netflix/vmaf/pull/726 this usage of the API should be valid.

Start by reproducing the out-of-order sequence 3970, 3974, 3972, 3973 and flushing before computing the pooled vmaf_neg score. Inspect the flush path and the API behavior described after PR #726; done means the final frame includes integer_motion2 and pooled scoring succeeds without failing.