Insert extra ARFs' in a gf group

Insert multiple arfs in a gf group to emulate multi-layer backward
reference frames structure. At maximum, two extra ARF's are inserted
in a gf group.

It improves the RD performance by 0.317% in Avg in lowres dataset.

Change-Id: I62c32e1b0f25b978484dd113b319bebcd959bf60
11 files changed