(container element for immediate play-out frame (IPF)) or
(container element for independent frame (IF)) does not indicate this frame is independent but
(value) indicates this frame is independent
ISO IEC 14496-12 (MPEG-4 ISO base media file format):
sample_number: "marking of the sync samples within the stream [...] If the sync sample box is not present, every sample is a sync sample"
group_description_index: "gives the index of the sample group entry which describes the samples in this group"
ISO IEC 14496-12:
Fix the muxer in order to correctly fill
stss with this frame number or
group_description_index value not
0 for this frame number, then remux the content.