Make SkBlitter hierarchy explicit about what needs to be implemented.

src/core/SkLinearBitmapPipeline_sample.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 SkLinearBitmapPipeline_sampler_DEFINED
 #define SkLinearBitmapPipeline_sampler_DEFINED
 
@@ skipping 410 matching lines @@
     // We're moving through source space faster than dst (zoomed out),
     // so we'll never reuse a source pixel or be able to do contiguous loads.
     void spanFastRate(Span span) {
         span_fallback(span, this);
     }
 
     Next* const                            fNext;
     PixelAccessor<colorType, colorProfile> fStrategy;
 };
 
+// -- BilerpSampler --------------------------------------------------------------------------------
 // BilerpSampler - use a bilerp filter to create runs of destination pixels.
 template<SkColorType colorType, SkColorProfileType colorProfile, typename Next>
 class BilerpSampler : public SkLinearBitmapPipeline::SampleProcessorInterface {
 public:
     template<typename... Args>
     BilerpSampler(SkLinearBitmapPipeline::BlendProcessorInterface* next, Args&& ... args)
         : fNext{next}, fStrategy{std::forward<Args>(args)...} { }
 
     BilerpSampler(SkLinearBitmapPipeline::BlendProcessorInterface* next,
                    const BilerpSampler& sampler)
@@ skipping 359 matching lines @@
         }
     }
 
     Next* const                            fNext;
     PixelAccessor<colorType, colorProfile> fStrategy;
 };
 
 }  // namespace
 
 #endif  // SkLinearBitmapPipeline_sampler_DEFINED