matroska: refactor code common to matroskademux and matroskaparse

Move the following functions to matroska-read-common.[ch] from
matroska-demux.c and matroska-parse.c:
    - gst_matroska_decode_content_encodings
    - gst_matroska_decompress_data

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