avformat/mux: Allow muxers to set custom min timestamp

Matroska requires pts to be >= 0 with a slight exception:
It has a mechanism to deal with codec delay, i.e. with
the data added at the beginning that does not correspond
to actual input data and should be discarded by the player.
Only the audio actually intended to be output needs to have
a timestamp >= 0.
In order to avoid unnecessary timestamp shifting, this patch
allows muxers to inform the shifting code about this so that
it can take it into account.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavformat/internal.h

@@ -310,6 +310,13 @@ typedef struct FFStream {
      */
     int64_t mux_ts_offset;
 
+    /**
+     * This is the lowest ts allowed in this track; it may be set by the muxer
+     * during init or write_header and influences the automatic timestamp
+     * shifting code.
+     */
+    int64_t lowest_ts_allowed;
+
     /**
      * Internal data to check for wrapping of the time stamp
      */