image/jpeg: support 4:1:1 and 4:1:0 chroma subsampling.

image/jpeg: support 4:1:1 and 4:1:0 chroma subsampling.

The test data was generated by:
convert video-001.png tmp.tga
cjpeg -quality 50 -sample 4x2,1x1,1x1 tmp.tga > video-001.q50.410.jpeg
cjpeg -quality 50 -sample 4x1,1x1,1x1 tmp.tga > video-001.q50.411.jpeg
cjpeg -quality 50 -sample 4x2,1x1,1x1 -progressive tmp.tga > video-001.q50.410.progressive.jpeg
cjpeg -quality 50 -sample 4x1,1x1,1x1 -progressive tmp.tga > video-001.q50.411.progressive.jpeg
rm tmp.tga

Change-Id: I5570389c462360f98c3160f3c6963d9466d511de
Reviewed-on: https://go-review.googlesource.com/6041
Reviewed-by: Rob Pike <r@golang.org>