avformat/mxfenc: do not write index tables with the same InstanceUID

Only index tables repeating previous index tables should use the same
InstaceUID. Use the index start position when generating the InstanceUID to fix
this.

Signed-off-by: Marton Balint <cus@passwd.hu>

libavformat/mxfenc.c

@@ -1760,7 +1760,7 @@ static void mxf_write_index_table_segment(AVFormatContext *s)
 
     // instance id
     mxf_write_local_tag(s, 16, 0x3C0A);
-    mxf_write_uuid(pb, IndexTableSegment, 0);
+    mxf_write_uuid(pb, IndexTableSegment, mxf->last_indexed_edit_unit);
 
     // index edit rate
     mxf_write_local_tag(s, 8, 0x3F0B);