image/gif: fix GIF encoding of sub-images.

benchmark                    old ns/op     new ns/op     delta
BenchmarkEncode              8641055       8646829       +0.07%

Fixes #7792.

LGTM=r
R=r
CC=dbathgate, golang-codereviews
https://golang.org/cl/147730043

src/image/gif/writer.go

@@ -233,10 +233,20 @@ func (e *encoder) writeImageBlock(pm *image.Paletted, delay int) {
 	e.writeByte(uint8(litWidth)) // LZW Minimum Code Size.
 
 	lzww := lzw.NewWriter(blockWriter{e: e}, lzw.LSB, litWidth)
-	_, e.err = lzww.Write(pm.Pix)
-	if e.err != nil {
-		lzww.Close()
-		return
+	if dx := b.Dx(); dx == pm.Stride {
+		_, e.err = lzww.Write(pm.Pix)
+		if e.err != nil {
+			lzww.Close()
+			return
+		}
+	} else {
+		for i, y := 0, b.Min.Y; y < b.Max.Y; i, y = i+pm.Stride, y+1 {
+			_, e.err = lzww.Write(pm.Pix[i : i+dx])
+			if e.err != nil {
+				lzww.Close()
+				return
+			}
+		}
 	}
 	lzww.Close()
 	e.writeByte(0x00) // Block Terminator.

src/image/gif/writer_test.go

@@ -102,6 +102,29 @@ func TestWriter(t *testing.T) {
 	}
 }
 
+func TestSubImage(t *testing.T) {
+	m0, err := readImg("../testdata/video-001.gif")
+	if err != nil {
+		t.Fatalf("readImg: %v", err)
+	}
+	m0 = m0.(*image.Paletted).SubImage(image.Rect(0, 0, 50, 30))
+	var buf bytes.Buffer
+	err = Encode(&buf, m0, nil)
+	if err != nil {
+		t.Fatalf("Encode: %v", err)
+	}
+	m1, err := Decode(&buf)
+	if err != nil {
+		t.Fatalf("Decode: %v", err)
+	}
+	if m0.Bounds() != m1.Bounds() {
+		t.Fatalf("bounds differ: %v and %v", m0.Bounds(), m1.Bounds())
+	}
+	if averageDelta(m0, m1) != 0 {
+		t.Fatalf("images differ")
+	}
+}
+
 var frames = []string{
 	"../testdata/video-001.gif",
 	"../testdata/video-005.gray.gif",