Add SkRasterPipeline blitter.

src/core/SkRasterPipeline.h

Index: src/core/SkRasterPipeline.h
diff --git a/src/core/SkRasterPipeline.h b/src/core/SkRasterPipeline.h
index 186ee654c206451f2c9831297f2d58afd8029d91..acbabcbadc0d7b17af5ab616e59591c62d2a61df 100644
--- a/src/core/SkRasterPipeline.h
+++ b/src/core/SkRasterPipeline.h
@@ -72,9 +72,10 @@ public:
 
     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
@@ -93,6 +94,9 @@ public:
         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>;