Take span len into account when determining whether gradient is vertical

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 878 matching lines @@
         const int rowIndex = (y & 1) << 1;
         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
+    if (SkScalarNearlyZero(dx * count)) { // gradient is vertical
         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);
         }
     }
 }