movenc: Don't write any tfdt atom for ismv files

The tfdt atom shouldn't be needed in those cases, we already
write tfxd atoms for ismv anyway, which is roughly equivalent.

This avoids having to declare the iso6 brand for ismv files.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/movenc.c

@@ -2583,7 +2583,8 @@ static int mov_write_traf_tag(AVIOContext *pb, MOVMuxContext *mov,
     ffio_wfourcc(pb, "traf");
 
     mov_write_tfhd_tag(pb, mov, track, moof_offset);
-    mov_write_tfdt_tag(pb, track);
+    if (mov->mode != MODE_ISM)
+        mov_write_tfdt_tag(pb, track);
     mov_write_trun_tag(pb, mov, track, moof_size);
     if (mov->mode == MODE_ISM) {
         mov_write_tfxd_tag(pb, track);
@@ -2758,7 +2759,7 @@ static int mov_write_ftyp_tag(AVIOContext *pb, AVFormatContext *s)
 
     // We add tfdt atoms when fragmenting, signal this with the iso6 compatible
     // brand. This is compatible with users that don't understand tfdt.
-    if (mov->flags & FF_MOV_FLAG_FRAGMENT)
+    if (mov->flags & FF_MOV_FLAG_FRAGMENT && mov->mode != MODE_ISM)
         ffio_wfourcc(pb, "iso6");
 
     if (mov->mode == MODE_3GP)