use pinned value during lerp for vertical gradients

src/effects/gradients/SkLinearGradient.cpp

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkLinearGradient.h"
 
 static const float kInv255Float = 1.0f / 255;
@@ skipping 709 matching lines @@
  *  TODOs
  *
  *  - tilemodes
  *  - interp before or after premul
  *  - perspective
  *  - optimizations
  *      - use fixed (32bit or 16bit) instead of floats?
  */
 
 static Sk4f lerp_color(float fx, const SkLinearGradient::LinearGradientContext::Rec* rec) {
+    SkASSERT(fx >= rec[0].fPos);
+    SkASSERT(fx <= rec[1].fPos);
+
     const float p0 = rec[0].fPos;
     const Sk4f c0 = rec[0].fColor;
     const Sk4f c1 = rec[1].fColor;
     const Sk4f diffc = c1 - c0;
     const float scale = rec[1].fPosScale;
     const float t = (fx - p0) * scale;
     return c0 + Sk4f(t) * diffc;
 }
 
 template <bool apply_alpha> void ramp(SkPMColor dstC[], int n, const Sk4f& c, const Sk4f& dc,
@@ skipping 147 matching lines @@
         dither0 = ditherCell[rowIndex];
         dither1 = ditherCell[rowIndex + 1];
         if (x & 1) {
             SkTSwap(dither0, dither1);
         }
     }
     const float dither[2] = { dither0, dither1 };
     const float invDx = 1 / dx;
 
     if (SkScalarNearlyZero(dx)) { // gradient is vertical
-        Sk4f c = lerp_color(fx, find_forward(fRecs.begin(), SkTPin(fx, 0.0f, 1.0f)));
+        const float pinFx = SkTPin(fx, 0.0f, 1.0f);
+        Sk4f c = lerp_color(pinFx, find_forward(fRecs.begin(), pinFx));
         if (fApplyAlphaAfterInterp) {
             fill<true>(dstC, count, c + dither0, c + dither1);
         } else {
             fill<false>(dstC, count, c + dither0, c + dither1);
         }
         return;
     }
 
     if (dx > 0) {
         if (fApplyAlphaAfterInterp) {
             this->shade4_dx_clamp<true, true>(dstC, count, fx, dx, invDx, dither);
         } else {
             this->shade4_dx_clamp<false, true>(dstC, count, fx, dx, invDx, dither);
         }
     } else {
         if (fApplyAlphaAfterInterp) {
             this->shade4_dx_clamp<true, false>(dstC, count, fx, dx, invDx, dither);
         } else {
             this->shade4_dx_clamp<false, false>(dstC, count, fx, dx, invDx, dither);
         }
     }
 }