Improved 2pass bitrate predictor. No real change most of the time, but allows correct ratecontrol on some pathological videos that used to diverge completely. Also improves prediction when 2nd pass bitrate is very different from 1st pass.
The new qscale2bits() has no simple inverse, so I also had to change rc_eq to output qscale instead of bits.


git-svn-id: svn://svn.videolan.org/x264/trunk@129 df754926-b1dd-0310-bc7b-ec298dee348c