When write multi-trun box, the MOV_TRUN_FIRST_SAMPLE_FLAGS flag
need judge by first param, not 0.
If the original video contains consecutive I frames,
this will cause the packets of fmp4 have error sample_flags ,
and then incorrect keyframes were generated,
and then error packet will be seeked.
Signed-off-by: Wang Yaqiang <wangyaqiang03@kuaishou.com>
Signed-off-by: Steven Liu <liuqi05@chinaffmpeg.org>