The testdata was generated by:
convert video-001.png tmp1.tga
cjpeg -rgb -sample 2x2,1x1,1x1 tmp1.tga > video-001.rgb.jpeg
djpeg -nosmooth -targa video-001.rgb.jpeg > tmp2.tga
convert tmp2.tga video-001.rgb.png
rm tmp1.tga tmp2.tga

Change-Id: I5da0591b9005c1c75e807311f157d385e0e20a38
Reviewed-on: https://go-review.googlesource.com/6910Reviewed-by: Rob Pike <r@golang.org>