movenc: Don't assume that fragment durations in pts is equal to duration in dts

For strict CFR, they should be pretty much equal, but if the stream
is VFR, there can be a sometimes significant difference.

Calculate the pts duration separately, used in sidx atoms and for
tfrf/tfxd boxes in smooth streaming ismv files.

Also make sure to reduce the duration of sidx entries according to
edit lists.

Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/movenc.h

@@ -105,6 +105,7 @@ typedef struct MOVTrack {
     int         tref_id; ///< trackID of the referenced track
     int64_t     start_dts;
     int64_t     start_cts;
+    int64_t     end_pts;
 
     int         hint_track;   ///< the track that hints this track, -1 if no hint track is set
     int         src_track;    ///< the track that this hint track describes