movenc: Set tfhd default sample flags based on actual samples, if possible

This avoids assuming that e.g. audio samples are marked as
sync samples.

This allows omitting the sample flags from trun, if the default
flags happen to be right for all the samples.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/movenc.c

@@ -2409,6 +2409,12 @@ static int mov_write_mfhd_tag(AVIOContext *pb, MOVMuxContext *mov)
     return 0;
 }
 
+static uint32_t get_sample_flags(MOVTrack *track, MOVIentry *entry)
+{
+    return entry->flags & MOV_SYNC_SAMPLE ? MOV_FRAG_SAMPLE_FLAG_DEPENDS_NO :
+           (MOV_FRAG_SAMPLE_FLAG_DEPENDS_YES | MOV_FRAG_SAMPLE_FLAG_IS_NON_SYNC);
+}
+
 static int mov_write_tfhd_tag(AVIOContext *pb, MOVMuxContext *mov,
                               MOVTrack *track, int64_t moof_offset)
 {
@@ -2454,22 +2460,21 @@ static int mov_write_tfhd_tag(AVIOContext *pb, MOVMuxContext *mov,
         track->default_size = -1;
 
     if (flags & MOV_TFHD_DEFAULT_FLAGS) {
-        track->default_sample_flags =
-            track->enc->codec_type == AVMEDIA_TYPE_VIDEO ?
-            (MOV_FRAG_SAMPLE_FLAG_DEPENDS_YES | MOV_FRAG_SAMPLE_FLAG_IS_NON_SYNC) :
-            MOV_FRAG_SAMPLE_FLAG_DEPENDS_NO;
+        /* Set the default flags based on the second sample, if available.
+         * If the first sample is different, that can be signaled via a separate field. */
+        if (track->entry > 1)
+            track->default_sample_flags = get_sample_flags(track, &track->cluster[1]);
+        else
+            track->default_sample_flags =
+                track->enc->codec_type == AVMEDIA_TYPE_VIDEO ?
+                (MOV_FRAG_SAMPLE_FLAG_DEPENDS_YES | MOV_FRAG_SAMPLE_FLAG_IS_NON_SYNC) :
+                MOV_FRAG_SAMPLE_FLAG_DEPENDS_NO;
         avio_wb32(pb, track->default_sample_flags);
     }
 
     return update_size(pb, pos);
 }
 
-static uint32_t get_sample_flags(MOVTrack *track, MOVIentry *entry)
-{
-    return entry->flags & MOV_SYNC_SAMPLE ? MOV_FRAG_SAMPLE_FLAG_DEPENDS_NO :
-           (MOV_FRAG_SAMPLE_FLAG_DEPENDS_YES | MOV_FRAG_SAMPLE_FLAG_IS_NON_SYNC);
-}
-
 static int mov_write_trun_tag(AVIOContext *pb, MOVMuxContext *mov,
                               MOVTrack *track, int moof_size)
 {