batchcomp: -h263_pb for pb frames
           -h263_iquant <num> to set intra quantiser
           -h263_sac	for syntax arithmetic coding
           -h263_obmc	for overlapped block motion compensation
           -h263_umv	for unrestricted MV mode (also turned on by obmc)
           -h263_df   deblocking filter
           -nokeys for chunks without I frames
           -t_setnumc   Override number of chunks to output
