image/png: speed up paletted encoding ~25%

Avoids a lot of redundant bounds checks.

R=nigeltao, rsc
CC=golang-dev
https://golang.org/cl/2678041

src/pkg/image/png/writer.go

@@ -311,9 +311,8 @@ func writeImage(w io.Writer, m image.Image, cb int) os.Error {
 				cr[0][3*x+3] = uint8(b >> 8)
 			}
 		case cbP8:
-			for x := b.Min.X; x < b.Max.X; x++ {
-				cr[0][x+1] = paletted.ColorIndexAt(x, y)
-			}
+			rowOffset := y * paletted.Stride
+			copy(cr[0][b.Min.X+1:], paletted.Pix[rowOffset+b.Min.X:rowOffset+b.Max.X])
 		case cbTCA8:
 			// Convert from image.Image (which is alpha-premultiplied) to PNG's non-alpha-premultiplied.
 			for x := b.Min.X; x < b.Max.X; x++ {

src/pkg/image/png/writer_test.go

@@ -5,6 +5,7 @@
 package png
 
 import (
+	"bytes"
 	"fmt"
 	"image"
 	"io"
@@ -68,3 +69,18 @@ func TestWriter(t *testing.T) {
 		}
 	}
 }
+
+func BenchmarkEncodePaletted(b *testing.B) {
+	b.StopTimer()
+	img := image.NewPaletted(640, 480,
+		[]image.Color{
+			image.RGBAColor{0, 0, 0, 255},
+			image.RGBAColor{255, 255, 255, 255},
+		})
+	b.StartTimer()
+	buffer := new(bytes.Buffer)
+	for i := 0; i < b.N; i++ {
+		buffer.Reset()
+		Encode(buffer, img)
+	}
+}