matroska: refactor code common to matroskademux and matroskaparse

Move the following function to matroska-read-common.[ch] from
matroska-demux.c and matroska-parse.c:
    - gst_matroska_{demux,parse}_peek_bytes

https://bugzilla.gnome.org/show_bug.cgi?id=650877
6 files changed