fftools/ffmpeg_mux: make ts_copy_start private to muxing code

It is no longer used outside of ffmpeg_mux*

fftools/ffmpeg_mux.h

@@ -60,6 +60,11 @@ typedef struct MuxStream {
     /* Threshold after which max_muxing_queue_size will be in effect */
     size_t muxing_queue_data_threshold;
 
+    // timestamp from which the streamcopied streams should start,
+    // in AV_TIME_BASE_Q;
+    // everything before it should be discarded
+    int64_t ts_copy_start;
+
     /* dts of the last packet sent to the muxer, in the stream timebase
      * used for making up missing dts values */
     int64_t last_mux_dts;