webp: allow VP8L + VP8X(with alpha)

Fixes golang/go#60437

Change-Id: I8d8ed597b7f92e136ab2af980b6b223235d8c90d
GitHub-Last-Rev: 5912bdb6fccecaab6106ad39f566cc60935b3387
GitHub-Pull-Request: golang/image#16
Reviewed-on: https://go-review.googlesource.com/c/image/+/498535
Reviewed-by: Nigel Tao <nigeltao@golang.org>
Reviewed-by: Nigel Tao <nigeltao@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>

testdata/gopher-doc.with-alpha.lossless.webp

diff --git a/testdata/gopher-doc.with-alpha.lossless.webp b/testdata/gopher-doc.with-alpha.lossless.webp
new file mode 100644
index 0000000..fe5018d
--- /dev/null
+++ b/testdata/gopher-doc.with-alpha.lossless.webp
Binary files differ

testdata/gopher-doc.with-alpha.png

diff --git a/testdata/gopher-doc.with-alpha.png b/testdata/gopher-doc.with-alpha.png
new file mode 100644
index 0000000..ba3236b
--- /dev/null
+++ b/testdata/gopher-doc.with-alpha.png
Binary files differ

webp/decode.go

diff --git a/webp/decode.go b/webp/decode.go
index 0df3c67..2371808 100644
--- a/webp/decode.go
+++ b/webp/decode.go
@@ -103,7 +103,7 @@
 			return m, image.Config{}, nil
 
 		case fccVP8L:
-			if wantAlpha || alpha != nil {
+			if alpha != nil {
 				return nil, image.Config{}, errInvalidFormat
 			}
 			if configOnly {

webp/decode_test.go

diff --git a/webp/decode_test.go b/webp/decode_test.go
index 00be03f..bb50ddd 100644
--- a/webp/decode_test.go
+++ b/webp/decode_test.go
@@ -177,6 +177,7 @@
 		"gopher-doc.2bpp",
 		"gopher-doc.4bpp",
 		"gopher-doc.8bpp",
+		"gopher-doc.with-alpha",
 		"tux",
 		"yellow_rose",
 	}