stream_filter: dash: add callback method on chunk download

modules/stream_filter/dash/http/Chunk.h

@@ -46,6 +46,7 @@ namespace dash
         {
             public:
                 Chunk           ();
+                virtual ~Chunk  () {}
 
                 size_t              getEndByte              () const;
                 size_t              getStartByte            () const;
@@ -71,6 +72,8 @@ namespace dash
                 void                setBitrate      (uint64_t bitrate);
                 int                 getBitrate      ();
 
+                virtual void        onDownload      () {}
+
             private:
                 std::string                 url;
                 std::string                 path;