Initial fix for -q 1 / mpeg-1 problems, some improvements to rate-control

Hi Steven, Linards,

I just checked in a fix for the quantization bug causing problems in at
low-q in mpeg-1 and for -q 1 in MPEG-2.

Experimentally I've also checked in some tuning of the look-ahead
rate-controller that should fix silly ooops causing excessive feedback
gain.    It *should* be more robust now.  Caveat: I haven't had time to
unpack some decent *long* test sequences. I'm intended to down-scale a
HD captures of "Howl's walking castle" and action-scenes from Pirates of
the Caribbean as a test but first I need a working script to generate
the YUV4MPEG and raw audio ;-)

Any favourites you have  gratefully accepted. 

cheers,

   Andrew



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