image/jpeg: correct and test reference slowFDCT and slowIDCT
The reference implementations slowFDCT and slowIDCT were not
rounding correctly, making the test not as good as it could be.
Before, the real implementations were required to always produce
values within ±2 of the reference; now, with no changes,
the real implementations produce values within ±1 of the (corrected)
reference.
Also tighten the test to return an error not just on a single value
exceeding tolerance but also on too many values at exactly that
tolerance.
Change-Id: I3dd6ca7582178fef972fb812d848f7a0158a6ed8
Reviewed-on: https://go-review.googlesource.com/c/go/+/705517
Auto-Submit: Russ Cox <rsc@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Alan Donovan <adonovan@google.com>