Implement multi-color-stops in linear gradients using Sk4f

src/effects/gradients/SkLinearGradient.h

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkLinearGradient_DEFINED
 #define SkLinearGradient_DEFINED
 
 #include "SkGradientShaderPriv.h"
+#include "SkNx.h"
 
 class SkLinearGradient : public SkGradientShaderBase {
 public:
     SkLinearGradient(const SkPoint pts[2], const Descriptor&);
 
     size_t contextSize() const override;
 
     class LinearGradientContext : public SkGradientShaderBase::GradientShaderBaseContext {
     public:
         LinearGradientContext(const SkLinearGradient&, const ContextRec&);
-        ~LinearGradientContext() {}
+        ~LinearGradientContext() override;
 
         void shadeSpan(int x, int y, SkPMColor dstC[], int count) override;
         void shadeSpan16(int x, int y, uint16_t dstC[], int count) override;
 
     private:
+        Sk4f*   fColors4;
+        float*  fPos;
+        float*  fPosScale;
+        int fColorCount;
+        void shadeSpan4(int x, int y, SkPMColor dstC[], int count);
+
         typedef SkGradientShaderBase::GradientShaderBaseContext INHERITED;
     };
 
     GradientType asAGradient(GradientInfo* info) const override;
 #if SK_SUPPORT_GPU
     const GrFragmentProcessor* asFragmentProcessor(GrContext*,
                                                    const SkMatrix& viewM,
                                                    const SkMatrix*,
                                                    SkFilterQuality) const override;
 #endif
 
     SK_TO_STRING_OVERRIDE()
     SK_DECLARE_PUBLIC_FLATTENABLE_DESERIALIZATION_PROCS(SkLinearGradient)
 
 protected:
     SkLinearGradient(SkReadBuffer& buffer);
     void flatten(SkWriteBuffer& buffer) const override;
     Context* onCreateContext(const ContextRec&, void* storage) const override;
 
 private:
     friend class SkGradientShader;
     typedef SkGradientShaderBase INHERITED;
     const SkPoint fStart;
     const SkPoint fEnd;
 };
 
 #endif