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/include/core/SkPaint.h b/include/core/SkPaint.h
index 7dc3a4e..ab93eac 100644
--- a/include/core/SkPaint.h
+++ b/include/core/SkPaint.h
@@ -943,6 +943,7 @@
             uintptr_t effects = reinterpret_cast<uintptr_t>(this->getLooper());
             effects |= reinterpret_cast<uintptr_t>(this->getMaskFilter());
             effects |= reinterpret_cast<uintptr_t>(this->getPathEffect());
+            effects |= reinterpret_cast<uintptr_t>(this->getImageFilter());
             if (!effects) {
                 return orig;
             }