Implement a computeFastBounds() traversal for SkImageFilter.

This allows for correct culling of primitives which have image filters applied.

R=reed@google.com
BUG=skia:

Review URL: https://codereview.chromium.org/137423005

git-svn-id: http://skia.googlecode.com/svn/trunk@13207 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/src/effects/SkBitmapSource.cpp b/src/effects/SkBitmapSource.cpp
index daf4fb0..4aab92f 100644
--- a/src/effects/SkBitmapSource.cpp
+++ b/src/effects/SkBitmapSource.cpp
@@ -78,3 +78,7 @@
     offset->fY = dstIRect.fTop;
     return true;
 }
+
+void SkBitmapSource::computeFastBounds(const SkRect&, SkRect* dst) const {
+    *dst = fDstRect;
+}