fix vertex alpha calculation in GrTesselator

src/gpu/GrTessellator.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "GrTessellator.h"
 
 #include "GrDefaultGeoProcFactory.h"
@@ skipping 186 matching lines @@
 
 inline void* emit_vertex(Vertex* v, const AAParams* aaParams, void* data) {
     if (!aaParams) {
         SkPoint* d = static_cast<SkPoint*>(data);
         *d++ = v->fPoint;
         return d;
     }
     if (aaParams->fTweakAlpha) {
         auto d = static_cast<GrDefaultGeoProcFactory::PositionColorAttr*>(data);
         d->fPosition = v->fPoint;
-        d->fColor = SkAlphaMulQ(aaParams->fColor, v->fAlpha);
+        d->fColor = SkAlphaMulQ(aaParams->fColor, SkAlpha255To256(v->fAlpha));
         d++;
         return d;
     }
     auto d = static_cast<GrDefaultGeoProcFactory::PositionColorCoverageAttr*>(data);
     d->fPosition = v->fPoint;
     d->fColor = aaParams->fColor;
     d->fCoverage = GrNormalizeByteToFloat(v->fAlpha);
     d++;
     return d;
 }
@@ skipping 1597 matching lines @@
         }
     }
     int actualCount = static_cast<int>(vertsEnd - *verts);
     SkASSERT(actualCount <= count);
     SkASSERT(pointsEnd - points == actualCount);
     delete[] points;
     return actualCount;
 }
 
 } // namespace