Basic version with only the system header and the program
stream map. An advanced version could include codec-specific
bits like SPS/PPS too. This is useful in connection with
e.g. multifilesink to make sure new files always start with
the stream headers.
header_length contains the length in bytes after the header_length
field, excluding the 6 byte start code and header_length field.
H.264 streams and some other formats need to be announced in the PSM.
VLC wouldn't play files created with mpegpsmux containing H.264 because
we claim the system header is larger than it actually is, which makes
VLC skip the program stream map which follows the system header, which
in turn makes it not recognise our H.264 video stream.
