compress/lzw: output a Clear code first, per GIF spec

The TestStartsWithClearCode test is new, but if it existed beforehand,
the want strings would be "\x81" and "Hi\x81" without a starting "\x80".

Fixes #26108
Fixes #33748
Updates makeworld-the-better-one/didder#7
Updates nothings/stb#1222

Change-Id: I35ac0ed862ba6ee921ba9aee257bc19828abaa82
Reviewed-on: https://go-review.googlesource.com/c/go/+/354710


Trust: Nigel Tao <nigeltao@golang.org>
Run-TryBot: Nigel Tao <nigeltao@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>