This makes the Go code match the libwebp C code's output on
blue-purple-pink-large.*-filter.lossy.webp
Also make the various WEBP benchmarks all decode a similar image,
the image at http://blog.golang.org/gophercon/image01.jpg, to make
it more meaningful to e.g. compare the simple filter's numbers with
the normal filter's numbers.
Also fix a "go vet" warning in webp/decode.go.
The test data was generated by:
wget http://blog.golang.org/gophercon/image01.jpg -O blue-purple-pink-large.jpeg
convert blue-purple-pink-large.jpeg blue-purple-pink-large.png
cwebp -lossless blue-purple-pink-large.png -o blue-purple-pink-large.lossless.webp
cwebp -q 80 -f 0 blue-purple-pink-large.png -o blue-purple-pink-large.no-filter.lossy.webp
cwebp -q 80 -strong blue-purple-pink-large.png -o blue-purple-pink-large.normal-filter.lossy.webp
cwebp -q 80 -nostrong blue-purple-pink-large.png -o blue-purple-pink-large.simple-filter.lossy.webp
dwebp -pgm blue-purple-pink-large.no-filter.lossy.webp -o tmp.pgm && convert tmp.pgm blue-purple-pink-large.no-filter.lossy.webp.ycbcr.png && rm tmp.pgm
dwebp -pgm blue-purple-pink-large.normal-filter.lossy.webp -o tmp.pgm && convert tmp.pgm blue-purple-pink-large.normal-filter.lossy.webp.ycbcr.png && rm tmp.pgm
dwebp -pgm blue-purple-pink-large.simple-filter.lossy.webp -o tmp.pgm && convert tmp.pgm blue-purple-pink-large.simple-filter.lossy.webp.ycbcr.png && rm tmp.pgm
LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/106230044
