Adds a new function to mpegts lib to create a iso639 language
descriptor from a language and use it in mpegtsmux to add
a language descriptor to audio streams that have a language set.
https://bugzilla.gnome.org/show_bug.cgi?id=763647
It was previously a mix and match of both variants, introducing just too much
confusion.
The prefix are from now on:
* GstMpegts for structures and type names (and not GstMpegTs)
* gst_mpegts_ for functions (and not gst_mpeg_ts_)
* GST_MPEGTS_ for enums/flags (and not GST_MPEG_TS_)
* GST_TYPE_MPEGTS_ for types (and not GST_TYPE_MPEG_TS_)
The rationale for chosing that is:
* the namespace is shorter/direct (it's mpegts, not mpeg_ts nor mpeg-ts)
* the namespace is one word under Gst
* it's shorter (yah)
The PES header length is calculated before setting the dynamic flags, returning
a wrong value. Small frames that should be sent in a single TS packet are
spawned to a new packet because of that error. For audio streams where a single
frame can cope in one TS packet it introduces a huge overhead.
For a 100B packet, we prepare a TS packet with a payload of(100+9)B. Then, we
write the TS header using this value in tsmux_write_ts_header, and call
tsmux_stream_get_data(). The dynamic flags where not set yet and now
tsmux_stream_pes_header_length() returns 14B instead of 9B. The payload of the
TS packet is 114B, 5B more than what was calculated. 109B are sent in a first
packet and the remaining 5B are sent in another one.
Fixes bug #628548.
