go.image/vp8: implement the normal filter.

The testdata was generated via:
cwebp foo.png -o foo.lossy.webp
dwebp -pgm foo.lossy.webp -o tmp.pgm
convert tmp.pgm foo.lossy.webp.ycbcr.png
rm tmp.pgm

TBR=r
R=r
CC=golang-codereviews
https://golang.org/cl/107330043
8 files changed