[Bug 790686] matroskamux: re-activate TOC support

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=790686

--- Comment #2 from fengalin at free.fr ---
These are the checks I did so far:

- Run the unit test (which w) to build a toc - chapters and sub-chapters -, mux
a testsrc to a file, demux it and compare the result with the original. I also
checked the temporary file with `gst-discoverer --toc`. 
- Use GStreamer based [media-toc](https://github.com/fengalin/media-toc) to
build tocs on real life files - including audio only - and check the results
with media-toc, ffmpeg and VLC.

Just a side note to share my observations with you: when I compared the
resulting binary to media originating from other frameworks, I noticed a couple
of potential improvements, but they don't break compatibility, so I decided not
to touch theses in the patch:

- The size field for the nested structures is first written to a default value
with the potential for the largest data size (the representation is 8 bytes
long), then updated when all the inner values are known. Whatever the actual
size of the resulting buffer, the field is kept 8 bytes long even for
structures that could use less. This is different from other files I inspected.
- The chapters titles end with a `\0` (this is explicit in
`gst_ebml_write_ascii`). While the specification is a little vague about this
("zero-padded when needed"), files originating from other frameworks dodn't use
this. IMHO, it's not required as the value is in a TLV.

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