Implement a computeFastBounds() traversal for SkImageFilter. This allows for correct culling of pri…

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 77 matching lines @@
     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);
     canvas.drawBitmap(src, 0, 0);
     *result = device->accessBitmap(false);
     offset->fX = bounds.fLeft;
     offset->fY = bounds.fTop;
     return true;
 }
+
+void SkDropShadowImageFilter::computeFastBounds(const SkRect& src, SkRect* dst) const {
+    if (getInput(0)) {
+        getInput(0)->computeFastBounds(src, dst);
+    } else {
+        *dst = src;
+    }
+
+    SkRect shadowBounds = *dst;
+    shadowBounds.offset(fDx, fDy);
+    shadowBounds.outset(SkScalarMul(fSigmaX, SkIntToScalar(3)),
+                        SkScalarMul(fSigmaY, SkIntToScalar(3)));
+    dst->join(shadowBounds);
+}
+