image/png: do not apply filters if level is NoCompression

PNG filters are applied to get better compression ratio.
It does not make sense to apply them if we are not going
to compress.

LGTM=nigeltao
R=nigeltao
CC=golang-codereviews
https://golang.org/cl/137830043

src/pkg/image/png/writer.go

@@ -417,7 +417,10 @@ func writeImage(w io.Writer, m image.Image, cb int, level int) error {
 		}
 
 		// Apply the filter.
-		f := filter(&cr, pr, bpp)
+		f := ftNone
+		if level != zlib.NoCompression {
+			f = filter(&cr, pr, bpp)
+		}
 
 		// Write the compressed bytes.
 		if _, err := zw.Write(cr[f]); err != nil {

src/pkg/image/png/writer_test.go

@@ -81,20 +81,23 @@ func TestWriterLevels(t *testing.T) {
 	m := image.NewNRGBA(image.Rect(0, 0, 100, 100))
 
 	var b1, b2 bytes.Buffer
-	var e1, e2 Encoder
-
-	if err := e1.Encode(&b1, m); err != nil {
+	if err := (&Encoder{}).Encode(&b1, m); err != nil {
 		t.Fatal(err)
 	}
-
-	e2.CompressionLevel = NoCompression
-	if err := e2.Encode(&b2, m); err != nil {
+	noenc := &Encoder{CompressionLevel: NoCompression}
+	if err := noenc.Encode(&b2, m); err != nil {
 		t.Fatal(err)
 	}
 
 	if b2.Len() <= b1.Len() {
 		t.Error("DefaultCompression encoding was larger than NoCompression encoding")
 	}
+	if _, err := Decode(&b1); err != nil {
+		t.Error("cannot decode DefaultCompression")
+	}
+	if _, err := Decode(&b2); err != nil {
+		t.Error("cannot decode NoCompression")
+	}
 }
 
 func TestSubImage(t *testing.T) {