Optimize BackwardReferenceHashChainFollowPath.

Instead of calling HashChainFindMethod, call a new (subset) method
HashChainFindOffset to get the offset/distance for a given length.

The encoding is tad faster at default compression

                       Before              After
                     bpp/rate            bpp/rate
442 Palette     0.2720/5.270 MP/s      0.2720/5.790 MP/s
558 non-palette 3.7607/0.797 MP/s      3.7607/0.816 MP/s

Change-Id: If4041a9c18f7e972f49fcbab8c3e2f013d8bf1cf