picture_chain: add vlc_picture_chain_PeekFront

In some rare case we may want to check the picture at the top of the chain/FIFO
without popping it.

include/vlc_picture.h

@@ -190,6 +190,18 @@ static inline picture_t * vlc_picture_chain_PopFront(picture_t **chain)
     return front;
 }
 
+/**
+ * Peek the front of a picture chain.
+ *
+ * The picture chain is unchanged.
+ *
+ * \return the front of the picture chain (the picture itself)
+ */
+static inline picture_t * vlc_picture_chain_PeekFront(picture_t **chain)
+{
+    return *chain;
+}
+
 /**
  * Append a picture to a picture chain.
  *