Fix a compression regression for images with long uniform regions.
Fix a compression regression for images with long uniform regions.

Change-Id: Id87a4ac2a22daaa71e8f3132e69703b9b3ddd752

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