Reorder reference frames optimally on second pass

Reorder reference frames optimally on second pass
About +0.1-0.2% compression at normal bitrates, up to +1% at very low bitrates.
Only works if the first pass uses the same number of refs as the second (i.e. not with fast first pass).
Thus, only worthwhile at insanely slow speeds: as such, enable slow-firstpass by default with preset placebo.
Note that this changes the stats file format!