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/SkDisplacementMapEffect.cpp b/src/effects/SkDisplacementMapEffect.cpp
index 6ad46ba..fc8d3dd 100644
--- a/src/effects/SkDisplacementMapEffect.cpp
+++ b/src/effects/SkDisplacementMapEffect.cpp
@@ -246,6 +246,14 @@
     return true;
 }
 
+void SkDisplacementMapEffect::computeFastBounds(const SkRect& src, SkRect* dst) const {
+    if (getColorInput()) {
+        getColorInput()->computeFastBounds(src, dst);
+    } else {
+        *dst = src;
+    }
+}
+
 ///////////////////////////////////////////////////////////////////////////////
 
 #if SK_SUPPORT_GPU