image/jpeg: clean up BenchmarkDecode and BenchmarkEncode to not

refer to opacity. Those references were copy/pasted from the
image/png encoding benchmarks, which cares whether or not the
source image is opaque, but the JPEG encoder does not care.

R=r
CC=golang-dev
https://golang.org/cl/6623052

src/pkg/image/jpeg/writer_test.go

@@ -171,7 +171,7 @@ func TestWriter(t *testing.T) {
 	}
 }
 
-func BenchmarkDecodeRGBOpaque(b *testing.B) {
+func BenchmarkDecode(b *testing.B) {
 	b.StopTimer()
 	data, err := ioutil.ReadFile("../testdata/video-001.jpeg")
 	if err != nil {
@@ -188,24 +188,21 @@ func BenchmarkDecodeRGBOpaque(b *testing.B) {
 	}
 }
 
-func BenchmarkEncodeRGBOpaque(b *testing.B) {
+func BenchmarkEncode(b *testing.B) {
 	b.StopTimer()
 	img := image.NewRGBA(image.Rect(0, 0, 640, 480))
-	// Set all pixels to 0xFF alpha to force opaque mode.
 	bo := img.Bounds()
 	rnd := rand.New(rand.NewSource(123))
 	for y := bo.Min.Y; y < bo.Max.Y; y++ {
 		for x := bo.Min.X; x < bo.Max.X; x++ {
-			img.Set(x, y, color.RGBA{
+			img.SetRGBA(x, y, color.RGBA{
 				uint8(rnd.Intn(256)),
 				uint8(rnd.Intn(256)),
 				uint8(rnd.Intn(256)),
-				255})
+				255,
+			})
 		}
 	}
-	if !img.Opaque() {
-		b.Fatal("expected image to be opaque")
-	}
 	b.SetBytes(640 * 480 * 4)
 	b.StartTimer()
 	options := &Options{Quality: 90}