add ImageShader, sharing code with its Bitmap cousin

src/core/SkBitmapProcShader.h

Index: src/core/SkBitmapProcShader.h
diff --git a/src/core/SkBitmapProcShader.h b/src/core/SkBitmapProcShader.h
index 63985c06c514472a2194ef498ed86c728c9ff1a7..fd8a5b7e20cc912a83a886da359ca2c933d8a17a 100644
--- a/src/core/SkBitmapProcShader.h
+++ b/src/core/SkBitmapProcShader.h
@@ -22,7 +22,7 @@ public:
 
     bool isOpaque() const override;
 
-    size_t contextSize() const override;
+    size_t contextSize() const override { return ContextSize(); }
 
     SK_TO_STRING_OVERRIDE()
     SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkBitmapProcShader)
@@ -33,11 +33,12 @@ public:
                                                    GrProcessorDataManager*) const override;
 #endif
 
+protected:
     class BitmapProcShaderContext : public SkShader::Context {
     public:
         // The context takes ownership of the state. It will call its destructor
         // but will NOT free the memory.
-        BitmapProcShaderContext(const SkBitmapProcShader&, const ContextRec&, SkBitmapProcState*);
+        BitmapProcShaderContext(const SkShader&, const ContextRec&, SkBitmapProcState*);
         ~BitmapProcShaderContext() override;
 
         void shadeSpan(int x, int y, SkPMColor dstC[], int count) override;
@@ -52,8 +53,7 @@ public:
 
         typedef SkShader::Context INHERITED;
     };
-
-protected:
+    
     void flatten(SkWriteBuffer&) const override;
     Context* onCreateContext(const ContextRec&, void* storage) const override;
     bool onIsABitmap(SkBitmap*, SkMatrix*, TileMode*) const override;
@@ -62,6 +62,12 @@ protected:
     uint8_t     fTileModeX, fTileModeY;
 
 private:
+    friend class SkImageShader;
+
+    static size_t ContextSize();
+    static Context* MakeContext(const SkShader&, TileMode tmx, TileMode tmy, const SkBitmap&,
+                                const ContextRec&, void* storage);
+
     typedef SkShader INHERITED;
 };