Add SkRasterPipeline blitter.

src/core/SkRasterPipeline.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 SkRasterPipeline_DEFINED
 #define SkRasterPipeline_DEFINED
 
@@ skipping 54 matching lines @@
 
         // It makes next() a good bit cheaper if we hold the next function to call here,
         // rather than logically simpler choice of the function implementing this stage.
         Fn fNext;
         void* fCtx;
     };
 
 
     SkRasterPipeline();
 
-    // Run the pipeline constructed with append(), walking x through [0,n),
+    // Run the pipeline constructed with append(), walking x through [x,x+n),
     // generally in 4 pixel steps, but sometimes 1 pixel at a time.
-    void run(size_t n);
+    void run(size_t x, size_t n);
+    void run(size_t n) { this->run(0, n); }
 
     // Use this append() if your stage is sensitive to the number of pixels you're working with:
     //   - body will always be called for a full 4 pixels
     //   - tail will always be called for a single pixel
     // Typically this is only an essential distintion for stages that read or write memory.
     void append(Fn body, const void* body_ctx,
                 Fn tail, const void* tail_ctx);
 
     // Most stages don't actually care if they're working on 4 or 1 pixel.
     void append(Fn fn, const void* ctx = nullptr) {
         this->append(fn, ctx, fn, ctx);
     }
 
     // Most 4 pixel or 1 pixel variants share the same context pointer.
     void append(Fn body, Fn tail, const void* ctx = nullptr) {
         this->append(body, ctx, tail, ctx);
     }
 
+    // Append all stages to this pipeline.
+    void extend(const SkRasterPipeline&);
+
 private:
     using Stages = SkSTArray<10, Stage, /*MEM_COPY=*/true>;
 
     // This no-op default makes fBodyStart and fTailStart unconditionally safe to call,
     // and is always the last stage's fNext as a sort of safety net to make sure even a
     // buggy pipeline can't walk off its own end.
     static void SK_VECTORCALL JustReturn(Stage*, size_t, Sk4f,Sk4f,Sk4f,Sk4f,
                                                          Sk4f,Sk4f,Sk4f,Sk4f);
 
     Stages fBody,
            fTail;
     Fn fBodyStart = &JustReturn,
        fTailStart = &JustReturn;
 };
 
 #endif//SkRasterPipeline_DEFINED