webp/nycbcra: deprecate package.

The standard image and image/color packages have NYCbCrA support, as of Go 1.6.

Change-Id: I3ed2a502b3b9fe720968ac21901115e20a11456a
Reviewed-on: https://go-review.googlesource.com/21066
Reviewed-by: Rob Pike <r@golang.org>

 webp/decode.go          | 5 ++---
 webp/nycbcra/nycbcra.go | 8 ++++++++
 2 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/webp/decode.go b/webp/decode.go
index 60fb556..134307e 100644
--- a/webp/decode.go
+++ b/webp/decode.go
@@ -18,7 +18,6 @@ import (
 	"golang.org/x/image/riff"
 	"golang.org/x/image/vp8"
 	"golang.org/x/image/vp8l"
-	"golang.org/x/image/webp/nycbcra"
 )
 
 var errInvalidFormat = errors.New("webp: invalid format")
@@ -98,7 +97,7 @@ func decode(r io.Reader, configOnly bool) (image.Image, image.Config, error) {
 				return nil, image.Config{}, err
 			}
 			if alpha != nil {
-				return &nycbcra.Image{
+				return &image.NYCbCrA{
 					YCbCr:   *m,
 					A:       alpha,
 					AStride: alphaStride,
@@ -138,7 +137,7 @@ func decode(r io.Reader, configOnly bool) (image.Image, image.Config, error) {
 			heightMinusOne = uint32(buf[7]) | uint32(buf[8])<<8 | uint32(buf[9])<<16
 			if configOnly {
 				return nil, image.Config{
-					ColorModel: nycbcra.ColorModel,
+					ColorModel: color.NYCbCrAModel,
 					Width:      int(widthMinusOne) + 1,
 					Height:     int(heightMinusOne) + 1,
 				}, nil
diff --git a/webp/nycbcra/nycbcra.go b/webp/nycbcra/nycbcra.go
index 2e1fe05..101c41f 100644
--- a/webp/nycbcra/nycbcra.go
+++ b/webp/nycbcra/nycbcra.go
@@ -4,6 +4,9 @@
 
 // Package nycbcra provides non-alpha-premultiplied Y'CbCr-with-alpha image and
 // color types.
+//
+// Deprecated: as of Go 1.6. Use the standard image and image/color packages
+// instead.
 package nycbcra // import "golang.org/x/image/webp/nycbcra"
 
 import (
@@ -11,6 +14,11 @@ import (
 	"image/color"
 )
 
+func init() {
+	println("The golang.org/x/image/webp/nycbcra package is deprecated, as of Go 1.6. " +
+		"Use the standard image and image/color packages instead.")
+}
+
 // TODO: move this to the standard image and image/color packages, so that the
 // image/draw package can have fast-path code. Moving would rename:
 //	nycbcra.Color      to color.NYCbCrA