 user is queried to
 specify the first rectangle.  rctE, denoting the object location in the
 last frame, can optionally be specified.  If rctE is given, the model
 histogram at fraction r of the video is (1-r)*histS+r*histE where histS
 and histE are the model histograms from the first and last frame.  If
 rctE==0 rectangle in final frame is queried, if rectE==-1 it is not used.

 Let T denote the length of the video. Returned values are of length t,
 where t==T if the object was tracked through the whole 