Revert of Revert of Extract most of the mutable state of SkShader into a separate Context object.

src/core/SkBlitter.h

Index: src/core/SkBlitter.h
diff --git a/src/core/SkBlitter.h b/src/core/SkBlitter.h
index d19a34badcd4d9ecbb78cd6f22f074f2ff0927d5..f76839e8b63c1ca3728f2902a3c83799772b4e2d 100644
--- a/src/core/SkBlitter.h
+++ b/src/core/SkBlitter.h
@@ -60,6 +60,13 @@
      *  returns false.
      */
     virtual bool isNullBlitter() const;
+
+    /**
+     *  Special methods for SkShaderBlitter. On all other classes this is a no-op.
+     */
+    virtual bool resetShaderContext(const SkBitmap& device, const SkPaint& paint,
+                                    const SkMatrix& matrix);
+    virtual SkShader::Context* getShaderContext() const;
 
     ///@name non-virtual helpers
     void blitMaskRegion(const SkMask& mask, const SkRegion& clip);