Remove isOpaque from SkSpecialImage, just use alpha type

src/core/SkSpecialImage.h

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file
  */
 
 #ifndef SkSpecialImage_DEFINED
 #define SkSpecialImage_DEFINED
 
@@ skipping 35 matching lines @@
     typedef void(*RasterReleaseProc)(void* pixels, ReleaseContext);
 
     const SkSurfaceProps& props() const { return fProps; }
 
     int width() const { return fSubset.width(); }
     int height() const { return fSubset.height(); }
     const SkIRect& subset() const { return fSubset; }
     SkColorSpace* getColorSpace() const;
 
     uint32_t uniqueID() const { return fUniqueID; }
-    virtual bool isOpaque() const { return false; }
+    virtual SkAlphaType alphaType() const = 0;
     virtual size_t getSize() const = 0;
 
     /**
      *  Ensures that a special image is backed by a texture (when GrContext is non-null). If no
      *  transformation is required, the returned image may be the same as this special image.
      *  If this special image is from a different GrContext, this will fail.
      */
     sk_sp<SkSpecialImage> makeTextureImage(GrContext*);
 
     /**
@@ skipping 73 matching lines @@
 
 private:
     const SkSurfaceProps fProps;
     const SkIRect        fSubset;
     const uint32_t       fUniqueID;
 
     typedef SkRefCnt INHERITED;
 };
 
 #endif