promote SkImage::AlphaType to SkAlphaType

include/core/SkAlpha.h

Index: include/core/SkAlpha.h
diff --git a/include/core/SkAlpha.h b/include/core/SkAlpha.h
new file mode 100644
index 0000000000000000000000000000000000000000..662b741a9518e00c805663aa00d74706f7c4db68
--- /dev/null
+++ b/include/core/SkAlpha.h
@@ -0,0 +1,50 @@
+/*
+ * Copyright 2013 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#ifndef SkAlpha_DEFINED
+#define SkAlpha_DEFINED
+
+#include "SkTypes.h"
+
+/**
+ *  Describes how to interpret the alpha compoent of a pixel.
+ */
+enum SkAlphaType {
+    /**
+     *  All pixels should be treated as opaque, regardless of the value stored
+     *  in their alpha field. Used for legacy images that wrote 0 or garbarge
+     *  in their alpha field, but intended the RGB to be treated as opaque.
+     */
+    kIgnore_SkAlphaType,
+
+    /**
+     *  All pixels are stored as opaque. This differs slightly from kIgnore in
+     *  that kOpaque has correct "opaque" values stored in the pixels, while
+     *  kIgnore may not, but in both cases the caller should treat the pixels
+     *  as opaque.
+     */
+    kOpaque_SkAlphaType,
+
+    /**
+     *  All pixels have their alpha premultiplied in their color components.
+     *  This is the natural format for the rendering target pixels.
+     */
+    kPremul_SkAlphaType,
+
+    /**
+     *  All pixels have their color components stored without any regard to the
+     *  alpha. e.g. this is the default configuration for PNG images.
+     *
+     *  This alpha-type is ONLY supported for input images. Rendering cannot
+     *  generate this on output.
+     */
+    kUnpremul_SkAlphaType,
+    
+    kLastEnum_SkAlphaType = kUnpremul_SkAlphaType
+};
+
+#endif