lavf: move AVStream.{nb_decoded_frames,mux_ts_offset} to AVStreamInternal

Those are private fields, no reason to have them exposed in a public
header.

libavformat/internal.h

@@ -225,6 +225,17 @@ struct AVStreamInternal {
 
     } *info;
 
+    /**
+     * Number of internally decoded frames, used internally in libavformat, do not access
+     * its lifetime differs from info which is why it is not in that structure.
+     */
+    int nb_decoded_frames;
+
+    /**
+     * Timestamp offset added to timestamps before muxing
+     */
+    int64_t mux_ts_offset;
+
     /**
      * Internal data to check for wrapping of the time stamp
      */