Speedup backward references.
Speedup backward references.

In case where the same offset is found in consecutive pixels,
the cost computation from one pixel can be re-used for the next.

Change-Id: Ic03c7d4ab95f3612eafc703349cfefd75273c3d7

PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp