tracking face before flandmark

[sthalik]

First use Haar.

Whichever works best:

• maybe use farneback, but only on small image part. Downscale (?), farneback is slow.
• maybe apply bbox to last flandmark result, then to the next... Frame deltas are small enough (?) Sometimes flandmark returns nonsense so take care of that too.
• maybe use HSV mask then meanshift. Regular meanshift sucks for face tracking, it deconverges all the time.
• track velocity of the bbox (?)
• apply flandmark several times on different bboxes (?) and use a heuristic to pick the good one. Similarity to last pose?