batchcomp: -h263_pb for pb frames
           -h263_iquant <num> to set intra quantiser
           -nokeys for chunks without I frames
           -t_setnumc   Override number of chunks to output
dec16:  amortised accurate 8->5 (19/9/96)
dec216Y5UV: Faster version (23/9/96)
dec246Y5UV:        ..
dec24: as before
decompress: slow version accurate 8->5
