Relocate the specialized linear gradient interval builder

src/effects/gradients/Sk4fLinearGradient.h

Index: src/effects/gradients/Sk4fLinearGradient.h
diff --git a/src/effects/gradients/Sk4fLinearGradient.h b/src/effects/gradients/Sk4fLinearGradient.h
index 863376087df61d2ed8372aa273657a54c4919e97..a0a09caf5cde500541ee7c3e9891bf25084b14b8 100644
--- a/src/effects/gradients/Sk4fLinearGradient.h
+++ b/src/effects/gradients/Sk4fLinearGradient.h
@@ -22,6 +22,9 @@ public:
 private:
     using INHERITED = GradientShaderBase4fContext;
 
+    void addMirrorIntervals(const SkLinearGradient&, const Sk4f& componentScale,
+                            bool dx_is_pos);
+
     template<typename DstType, TileMode>
     class LinearIntervalProcessor;
 
@@ -33,6 +36,10 @@ private:
 
     template <typename DstType, bool premul, SkShader::TileMode tileMode>
     void shadeSpanInternal(int x, int y, DstType[], int count) const;
+
+    const Interval* findInterval(SkScalar fx) const;
+
+    mutable const Interval*      fCachedInterval;
 };
 
 #endif // Sk4fLinearGradient_DEFINED