Return fence to client in dequeuBuffer

Change-Id: Ic19750793ad993f0e85391f3cabd743ba565d4c9
diff --git a/libs/ui/Fence.cpp b/libs/ui/Fence.cpp
index 5c17d10..08340f2 100644
--- a/libs/ui/Fence.cpp
+++ b/libs/ui/Fence.cpp
@@ -62,6 +62,10 @@
     return sp<Fence>(new Fence(result));
 }
 
+int Fence::dup() const {
+    return ::dup(mFenceFd);
+}
+
 size_t Fence::getFlattenedSize() const {
     return 0;
 }