Add alphaType() to SkImage

gm/image.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include <functional>
 #include "gm.h"
 #include "SkAutoPixmapStorage.h"
@@ skipping 285 matching lines @@
 private:
     typedef skiagm::GM INHERITED;
 };
 DEF_GM( return new ScalePixelsGM; )
 
 ///////////////////////////////////////////////////////////////////////////////////////////////////
 
 #include "SkImageGenerator.h"
 
 static SkImageInfo make_info(SkImage* img) {
-    return SkImageInfo::MakeN32(img->width(), img->height(),
-                                img->isOpaque() ? kOpaque_SkAlphaType : kPremul_SkAlphaType);
+    return SkImageInfo::MakeN32(img->width(), img->height(), img->alphaType());
 }
 
 // Its simple, but I wonder if we should expose this formally?
 //
 class ImageGeneratorFromImage : public SkImageGenerator {
 public:
     ImageGeneratorFromImage(SkImage* img) : INHERITED(make_info(img)), fImg(SkRef(img)) {}
 
 protected:
     bool onGetPixels(const SkImageInfo& info, void* pixels, size_t rowBytes, SkPMColor ctable[],
@@ skipping 183 matching lines @@
         }
         if (context) {
             sk_sp<SkImage> texImage(image->makeTextureImage(context));
             if (texImage) {
                 canvas->drawImage(texImage, 0, 0);
             }
         }
         canvas->translate(image->width() + kPad, 0);
     }
 }