- disable frame.pts only mp4v (seems to be bug concerning only that
  codec)
- initialize p_sys->i_hq to FF_MB_DECISION_RD if else ain't specified
  (seems like it's used uninitialised otherwise)
- set mb_lmin/mb_lmax and lmin/lmax also with qmin (ffmpeg documentation
  says lmax = qmax * FF_QP2LAMBDA) seems to help rate-control.