[Sk4fGradient] Use infinity floats for clamp-mode extreme positions

src/effects/gradients/Sk4fLinearGradient.cpp

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "Sk4fLinearGradient.h"
 #include "Sk4x4f.h"
 #include "SkXfermode.h"
@@ skipping 304 matching lines @@
             advX = this->advance_interval(advX);
         }
         SkASSERT(advX < fAdvX);
 
         fCc = fCc + fDcDx * Sk4f(advX);
         fAdvX -= advX;
     }
 
 private:
     void compute_interval_props(SkScalar t) {
-        const Sk4f dC = DstTraits<dstType, premul>::load(fInterval->fDc);
+        fZeroRamp     = fIsVertical || fInterval->isZeroRamp();
         fCc           = DstTraits<dstType, premul>::load(fInterval->fC0);
-        fCc           = fCc + dC * Sk4f(t);
-        fDcDx         = dC * fDx;
-        fZeroRamp     = fIsVertical || fInterval->isZeroRamp();
+
+        if (fInterval->isZeroRamp()) {
+            fDcDx = 0;
+        } else {
+            const Sk4f dC = DstTraits<dstType, premul>::load(fInterval->fDc);
+            fCc           = fCc + dC * Sk4f(t);
+            fDcDx         = dC * fDx;
+        }
     }
 
     const Interval* next_interval(const Interval* i) const {
         SkASSERT(i >= fFirstInterval);
         SkASSERT(i <= fLastInterval);
         i++;
 
         if (tileMode == kClamp_TileMode) {
             SkASSERT(i <= fLastInterval);
             return i;
@@ skipping 134 matching lines @@
         static_cast<const LinearGradient4fContext*>(state->fCtx);
 
     if (ctx->fColorsArePremul) {
         ctx->shadePremulSpan<DstType::F16, ApplyPremul::False>(
             x, y, dst.writable_addr64(x, y), count);
     } else {
         ctx->shadePremulSpan<DstType::F16, ApplyPremul::True>(
             x, y, dst.writable_addr64(x, y), count);
     }
 }