Add specialized rgba8888 unit sampler.

src/core/SkLinearBitmapPipeline.h

Index: src/core/SkLinearBitmapPipeline.h
diff --git a/src/core/SkLinearBitmapPipeline.h b/src/core/SkLinearBitmapPipeline.h
index e304ef85e718ef0407539940502eca0ea2d699c1..32e464100fc2144696193d77a5547ece56befd02 100644
--- a/src/core/SkLinearBitmapPipeline.h
+++ b/src/core/SkLinearBitmapPipeline.h
@@ -62,6 +62,7 @@ public:
     class PointProcessorInterface;
     class SampleProcessorInterface;
     class PixelPlacerInterface;
+    class DestinationInterface;
 
     // These values were generated by the assert above in PolymorphicUnion.
     using MatrixStage = PolymorphicUnion<PointProcessorInterface, 160>;
@@ -75,6 +76,7 @@ private:
     TileStage   fTiler;
     SampleStage fSampleStage;
     PixelStage  fPixelStage;
+    DestinationInterface* fLastStage;
 };
 
 #endif  // SkLinearBitmapPipeline_DEFINED