image/jpeg: initialize dct_test constants at compile time
Doing so is slightly more accurate than calculating at run time (because
of float64 rounding errors): https://go.dev/play/p/hrOzHDLjd5K
Having these more accurate values isn't necessary for tests to pass, but
it's helpful if doing printf-debugging or stepping through the code.
Change-Id: I07a65678936e4db05b11f9d8d952b32b2acd51a8
Reviewed-on: https://go-review.googlesource.com/c/go/+/624716 Reviewed-by: Dmitri Shuralyov <dmitshur@google.com> Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org> Reviewed-by: Nigel Tao <nigeltao@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>