fftools/ffmpeg: use a separate variable for discontinuity offset

This will allow to move normal offset handling to demuxer thread, since
discontinuities currently have to be processed in the main thread, as
the code uses some decoder-produced values.

fftools/ffmpeg.h

@@ -427,6 +427,10 @@ typedef struct InputFile {
     int input_sync_ref;
 
     int64_t ts_offset;
+    /**
+     * Extra timestamp offset added by discontinuity handling.
+     */
+    int64_t ts_offset_discont;
     int64_t last_ts;
     int64_t start_time;   /* user-specified start time in AV_TIME_BASE or AV_NOPTS_VALUE */
     int64_t recording_time;