image/jpeg: support decoding CMYK and YCbCrK images.

image/jpeg: support decoding CMYK and YCbCrK images.

The new testdata was created by:

convert video-001.png -colorspace cmyk video-001.cmyk.jpeg

video-001.cmyk.jpeg was then converted back to video-001.cmyk.png via
the GIMP. ImageMagick (convert) wasn't used for this second conversion
because IM's default color profiles complicates things.

Fixes #4500.

Change-Id: Ibf533f6a6c7e76883acc493ce3a4289d7875df3f
Reviewed-on: https://go-review.googlesource.com/4801
Reviewed-by: Rob Pike <r@golang.org>