image/gif: allow encoding a single-frame image whose top-left corner

isn't (0, 0).

Also fix a s/b.Min.X/b.Max.X/ typo in bounds checking.

Fixes #10676

Change-Id: Ie5ff7ec20ca30367a8e65d32061959a2d8e089e9
Reviewed-on: https://go-review.googlesource.com/9712


Reviewed-by: Rob Pike <r@golang.org>