matroskademux: Allow Matroska headers to be read more than once

matroskademux: Allow Matroska headers to be read more than once

This is necessary for MSE, where a new MSE initialization segment may be
appended at any point. These MSE initialization segments consist of an
entire WebM file until the first Cluster element (not included). [1]

Note that track definitions are ignored on successive headers, they must
match, but this is not checked by matroskademux (look for
`(!demux->tracks_parsed)` in the code).

Source pads are not altered when the new headers are read.

This patch has been splitted from the original patch from eocanha in [2].

[1] https://www.w3.org/TR/mse-byte-stream-format-webm/
[2] https://bug334082.bugzilla-attachments.gnome.org/attachment.cgi?id=362212

https://bugzilla.gnome.org/show_bug.cgi?id=793333