MPEG4 muxer can create fragmented streams if one set (by "or" operation) FLAG_FRAGMENTED_AT_I_PICTURES to
m_lFlags variable in MuxerParams. The stream being created is written by fragments then. Each fragment will keep
frames from one GOP.
The other possibility is to use FLAG_FRAGMENTED_BY_HEADER_SIZE. Here the muxer collects frames until generated header is greater then MAX_MOOF_SIZE which is equal to 200000 and hardcoded in umc_mp4_mux_defs.h. if this condition is true then starting from the nearest I frame the next fragment is started and the current one is ready for playback.