Change the B-frame coding structure.

Originally we can have a BRF right before an overlay frame (in
display order), which might be unnecessary since we already has a
quality backward reference frame (ARF).
This patch avoids such a coding structure and improves the RD
performance by 0.086% in Avg in the lowres dataset, and 0.153 in
Avg in the midres dataset.

In the lowres dataset, significant gains are obtained for the
following sequences:

mobisode2_240p: 0.563%
keiba_240p: 0.440%
bus_cif: 0.336%
soccer_cif: 0.333%

And the performance drops only in the following four video sequences:

motherdaughter_cif: 0.028%
bqsquare_240p: 0.017%
basketballpass_240p: 0.015%
bowing_cif: 0.006%

Change-Id: Ic94f648ba8e52eb0014933d484fb247610a9ae05