[release-branch.go1] image/jpeg: fix quantization tables to be in zig-zag order, not natural

««« backport 292816148e44
image/jpeg: fix quantization tables to be in zig-zag order, not natural
order.

JPEG images generated prior to this CL are still valid JPEGs, as the
quantization tables used are encoded in the wire format. Such JPEGs just
don't use the recommended quantization tables.

R=r, dsymonds, raph, adg
CC=golang-dev, tuom.larsen
https://golang.org/cl/6497083

»»»