SkRasterPipeline: simplify impl and remove need to rewire stages

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 78 matching lines @@
     }
 
     // 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);
     }
 
 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;
-    bool   fReadyToRun = false;
+    Fn fBodyStart = &JustReturn,
+       fTailStart = &JustReturn;
 };
 
 #endif//SkRasterPipeline_DEFINED