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() {}
 
         void shadeSpan(int x, int y, SkPMColor dstC[], int count) override;
         void shadeSpan16(int x, int y, uint16_t dstC[], int count) override;
 
+        struct Rec {
+            Sk4f    fColor;
+            float   fPos;
+            float   fPosScale;
+        };
     private:
+        SkTDArray<Rec>  fRecs;
+        bool            fApplyAlphaAfterInterp;
+
+        void shade4_clamp(int x, int y, SkPMColor dstC[], int count);
+        template <bool> void shade4_pos_clamp(SkPMColor dstC[], int count, float fx, float dx,
+                                              float invDx, Sk4f dither0, Sk4f dither1);
+        template <bool> void shade4_neg_clamp(SkPMColor dstC[], int count, float fx, float dx,
+                                              float invDx, Sk4f dither0, Sk4f dither1);
+
         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