image: use three-index slice for NewYCbCr.

This ensures that changing an image.YCbCr's Y values can't change its
chroma values, even after re-slicing up to capacity.

Change-Id: Icb626561522e336a3220e10f456c95330ae7db9e
Reviewed-on: https://go-review.googlesource.com/2209Reviewed-by: Rob Pike <r@golang.org>

src/image/ycbcr.go

@@ -144,11 +144,14 @@ func NewYCbCr(r Rectangle, subsampleRatio YCbCrSubsampleRatio) *YCbCr {
 		cw = w
 		ch = h
 	}
-	b := make([]byte, w*h+2*cw*ch)
+	i0 := w*h + 0*cw*ch
+	i1 := w*h + 1*cw*ch
+	i2 := w*h + 2*cw*ch
+	b := make([]byte, i2)
 	return &YCbCr{
-		Y:              b[:w*h],
-		Cb:             b[w*h+0*cw*ch : w*h+1*cw*ch],
-		Cr:             b[w*h+1*cw*ch : w*h+2*cw*ch],
+		Y:              b[:i0:i0],
+		Cb:             b[i0:i1:i1],
+		Cr:             b[i1:i2:i2],
 		SubsampleRatio: subsampleRatio,
 		YStride:        w,
 		CStride:        cw,

src/image/ycbcr_test.go

@@ -105,3 +105,27 @@ func testYCbCr(t *testing.T, r Rectangle, subsampleRatio YCbCrSubsampleRatio, de
 		}
 	}
 }
+
+func TestYCbCrSlicesDontOverlap(t *testing.T) {
+	m := NewYCbCr(Rect(0, 0, 8, 8), YCbCrSubsampleRatio420)
+	names := []string{"Y", "Cb", "Cr"}
+	slices := [][]byte{
+		m.Y[:cap(m.Y)],
+		m.Cb[:cap(m.Cb)],
+		m.Cr[:cap(m.Cr)],
+	}
+	for i, slice := range slices {
+		want := uint8(10 + i)
+		for j := range slice {
+			slice[j] = want
+		}
+	}
+	for i, slice := range slices {
+		want := uint8(10 + i)
+		for j, got := range slice {
+			if got != want {
+				t.Fatalf("m.%s[%d]: got %d, want %d", names[i], j, got, want)
+			}
+		}
+	}
+}