The testdata was generated by:
convert video-001.png tmp1.tga
cjpeg -quality 100 -sample 2x2,1x2,1x2 tmp1.tga > video-001.221212.jpeg
djpeg -nosmooth -targa video-001.221212.jpeg > tmp2.tga
convert tmp2.tga video-001.221212.png
rm tmp1.tga tmp2.tga

Change-Id: Ica241dfc19b3eb47ade150bf0432373c6006c38a
Reviewed-on: https://go-review.googlesource.com/7264Reviewed-by: Rob Pike <r@golang.org>