add ImageShader, sharing code with its Bitmap cousin

src/image/SkImage_Gpu.h

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkImage_Gpu_DEFINED
 #define SkImage_Gpu_DEFINED
 
@@ skipping 22 matching lines @@
             tex->resourcePriv().makeBudgeted();
         } else {
             tex->resourcePriv().makeUnbudgeted();
         }
     }
 
     bool getROPixels(SkBitmap*) const override;
     GrTexture* asTextureRef(GrContext* ctx, SkImageUsageType usage) const override;
 
     GrTexture* peekTexture() const override { return fTexture; }
-    SkShader* onNewShader(SkShader::TileMode,
-                          SkShader::TileMode,
-                          const SkMatrix* localMatrix) const override;
     bool isOpaque() const override;
     SkSurface* onNewSurface(const SkImageInfo&, const SkSurfaceProps&) const override;
     bool onReadPixels(const SkImageInfo&, void* dstPixels, size_t dstRowBytes,
                       int srcX, int srcY) const override;
 
 private:
     SkAutoTUnref<GrTexture>     fTexture;
     const int                   fSampleCountForNewSurfaces;   // 0 if we don't know
     const SkAlphaType           fAlphaType;
     const SkSurface::Budgeted   fBudgeted;
     mutable SkAtomic<bool>      fAddedRasterVersionToCache;
 
 
     typedef SkImage_Base INHERITED;
 };
 
 #endif