Clamp negative sigma to 0 in SkDropShadowImageFilter.

src/effects/SkDropShadowImageFilter.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkDropShadowImageFilter.h"
 
 #include "SkBitmap.h"
@@ skipping 61 matching lines @@
     }
 
     SkAutoTUnref<SkBaseDevice> device(proxy->createDevice(bounds.width(), bounds.height()));
     if (NULL == device.get()) {
         return false;
     }
     SkCanvas canvas(device.get());
 
     SkVector sigma, localSigma = SkVector::Make(fSigmaX, fSigmaY);
     matrix.mapVectors(&sigma, &localSigma, 1);
+    sigma.fX = SkMaxScalar(0, sigma.fX);
+    sigma.fY = SkMaxScalar(0, sigma.fY);
     SkAutoTUnref<SkImageFilter> blurFilter(new SkBlurImageFilter(sigma.fX, sigma.fY));
     SkAutoTUnref<SkColorFilter> colorFilter(SkColorFilter::CreateModeFilter(fColor, SkXfermode::kSrcIn_Mode));
     SkPaint paint;
     paint.setImageFilter(blurFilter.get());
     paint.setColorFilter(colorFilter.get());
     paint.setXfermodeMode(SkXfermode::kSrcOver_Mode);
     SkVector offsetVec, localOffsetVec = SkVector::Make(fDx, fDy);
     matrix.mapVectors(&offsetVec, &localOffsetVec, 1);
     canvas.translate(-SkIntToScalar(bounds.fLeft), -SkIntToScalar(bounds.fTop));
     canvas.drawBitmap(src, offsetVec.fX, offsetVec.fY, &paint);
@@ skipping 11 matching lines @@
         *dst = src;
     }
 
     SkRect shadowBounds = *dst;
     shadowBounds.offset(fDx, fDy);
     shadowBounds.outset(SkScalarMul(fSigmaX, SkIntToScalar(3)),
                         SkScalarMul(fSigmaY, SkIntToScalar(3)));
     dst->join(shadowBounds);
 }