Current commit is just an API template  for the rest of the code, and
I will add inner logic later.

Altref  frames  generate a  lot  of  bitrate  and  at the  same  time
other  frames  refer to  them  a  lot, so  it  makes  sense to  apply
special  compensation-based adaptive  quantization scheme  for altref
frames. E.g.,  for blocks  that are  good predictors  for the  future
apply rate-control  chosen quantizer  while for bad  predictors apply
worse one.

Change-Id: Iba3f8ec349470673b7249f6a125f6859336a47c8