// matroska "Low Overhead Bitstream Format syntax" format
// "Each Block contains one Temporal Unit containing one or more OBUs. Each OBU stored in the Block MUST contain its header and its payload."
// "The OBUs in the Block follow the [Low Overhead Bitstream Format syntax]. They MUST have the [obu_has_size_field] set to 1 except for the last OBU in the frame, for which [obu_has_size_field] MAY be set to 0, in which case it is assumed to fill the remainder of the frame."