font/plan9font: fix byteoffset for non-zero origin

Updates golang/go#57258

Change-Id: I4c7305c8e61f2d2cc0227c0daf0b003bbc466497
Reviewed-on: https://go-review.googlesource.com/c/image/+/458035
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Nigel Tao <nigeltao@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Nigel Tao (INACTIVE; USE @golang.org INSTEAD) <nigeltao@google.com>

font/plan9font/plan9font.go

@@ -370,6 +370,8 @@ type plan9Image struct {
 }
 
 func (m *plan9Image) byteoffset(x, y int) int {
+	x -= m.rect.Min.X
+	y -= m.rect.Min.Y
 	a := y * m.width
 	if m.depth < 8 {
 		// We need to always round down, but Go rounds toward zero.