move SkImageInfo into its own header

include/core/SkAlpha.h

-/*
- * 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
-};
-
-static inline bool SkAlphaTypeIsOpaque(SkAlphaType at) {
-    return (unsigned)at <= kOpaque_SkAlphaType;
-}
-#endif