[x265] sao: refine sao merge mode

[Steve Borho]

On 10/15, Satoshi Nakagawa wrote:
> # HG changeset patch
> # User Satoshi Nakagawa <nakagawa424 at oki.com>
> # Date 1413341637 -32400
> #      Wed Oct 15 11:53:57 2014 +0900
> # Node ID 8014e8d2c321148c7d68942b6f4552b8eede6e1c
> # Parent  02ff8eaad63232e958153e8b7cdcd5907141a7b6
> sao: refine sao merge mode

I hand-rebased these changes on top of the latest tip.  Let me know if
you spot any problems with it. It hasn't changed any outputs that I've
detected.

-- 
Steve Borho