image/draw: fix golden test for YCbCr.RGBA change.

The previous change was
https://go-review.googlesource.com/#/c/8073/

Change-Id: I0c48502d1ba90fb5d41c5e66346a8e7f4ee87ce7
Reviewed-on: https://go-review.googlesource.com/8151Reviewed-by: Nigel Tao <nigeltao@golang.org>

src/image/draw/draw_test.go

@@ -163,8 +163,8 @@ var drawTests = []drawTest{
 	// The source pixel is {0, 0, 136} in YCbCr-space, which is {11, 38, 0, 255} in RGB-space.
 	{"ycbcr", vgradCr(), fillAlpha(255), Over, color.RGBA{11, 38, 0, 255}},
 	{"ycbcrSrc", vgradCr(), fillAlpha(255), Src, color.RGBA{11, 38, 0, 255}},
-	{"ycbcrAlpha", vgradCr(), fillAlpha(192), Over, color.RGBA{42, 28, 0, 255}},
-	{"ycbcrAlphaSrc", vgradCr(), fillAlpha(192), Src, color.RGBA{8, 28, 0, 192}},
+	{"ycbcrAlpha", vgradCr(), fillAlpha(192), Over, color.RGBA{42, 29, 0, 255}},
+	{"ycbcrAlphaSrc", vgradCr(), fillAlpha(192), Src, color.RGBA{8, 29, 0, 192}},
 	{"ycbcrNil", vgradCr(), nil, Over, color.RGBA{11, 38, 0, 255}},
 	{"ycbcrNilSrc", vgradCr(), nil, Src, color.RGBA{11, 38, 0, 255}},
 	// Uniform mask (100%, 75%, nil) and variable Gray source.