reuse motion vectors/mb types/field select values of the source video, if the SSE for a macroblock which is predicted with these values is below me_threshold
currently works only with mpeg1/2 source or some luck
may need -sync 0 as otherwise framedrops could lead to extreemly long b frame sequences

Originally committed as revision 3042 to svn://svn.ffmpeg.org/ffmpeg/trunk