Take span len into account when determining whether gradient is vertical

src/effects/gradients/SkLinearGradient.cpp

Index: src/effects/gradients/SkLinearGradient.cpp
diff --git a/src/effects/gradients/SkLinearGradient.cpp b/src/effects/gradients/SkLinearGradient.cpp
index bfa473c0123eefa1d2831953513eff65700bf8b8..f47b6ab30e7fa81392abd02afe9a5a89f38d687c 100644
--- a/src/effects/gradients/SkLinearGradient.cpp
+++ b/src/effects/gradients/SkLinearGradient.cpp
@@ -896,7 +896,7 @@ void SkLinearGradient::LinearGradientContext::shade4_clamp(int x, int y, SkPMCol
     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) {