Implement correct clipping for image filters.

src/effects/SkDropShadowImageFilter.cpp

Index: src/effects/SkDropShadowImageFilter.cpp
diff --git a/src/effects/SkDropShadowImageFilter.cpp b/src/effects/SkDropShadowImageFilter.cpp
index 0bc7e1211f26d0e2523e80f49a39943e93de1f27..7cb515265c2151509cfd18d630ca7f8873780be8 100644
--- a/src/effects/SkDropShadowImageFilter.cpp
+++ b/src/effects/SkDropShadowImageFilter.cpp
@@ -112,3 +112,22 @@ void SkDropShadowImageFilter::computeFastBounds(const SkRect& src, SkRect* dst)
                         SkScalarMul(fSigmaY, SkIntToScalar(3)));
     dst->join(shadowBounds);
 }
+
+bool SkDropShadowImageFilter::onFilterBounds(const SkIRect& src, const SkMatrix& ctm,
+                                             SkIRect* dst) const {
+    SkIRect bounds = src;
+    if (getInput(0) && !getInput(0)->filterBounds(src, ctm, &bounds)) {
+        return false;
+    }
+    SkVector offsetVec, localOffsetVec = SkVector::Make(fDx, fDy);
+    ctm.mapVectors(&offsetVec, &localOffsetVec, 1);
+    bounds.offset(-SkScalarCeilToInt(offsetVec.x()),
+                  -SkScalarCeilToInt(offsetVec.y()));
+    SkVector sigma, localSigma = SkVector::Make(fSigmaX, fSigmaY);
+    ctm.mapVectors(&sigma, &localSigma, 1);
+    bounds.outset(SkScalarCeilToInt(SkScalarMul(sigma.x(), SkIntToScalar(3))),
+                  SkScalarCeilToInt(SkScalarMul(sigma.y(), SkIntToScalar(3))));
+    bounds.join(src);
+    *dst = bounds;
+    return true;
+}