Smarter LZ77 for uniform regions.
Smarter LZ77 for uniform regions.

No need to find backward references for pixels in uniform regions
by looking at all pixels.
Only pixels at the same distance from the end need to be compared to.

Change-Id: I4f187e965f0667d3a929775726a412f7e69f6473

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