matroska: refactor code common to matroskademux and matroskaparse

Replace the following functions with their gst_matroska_read_common_*
counterparts:
    - gst_matroska_{demux,parse}_parse_index
    - gst_matroska_{demux,parse}_parse_skip
    - gst_matroska_{demux,parse}_stream_from_num

Introduce GstMatroskaReadCommon to contain those members of
GstMatroskaDemux and GstMatroskaParse that were used by the above
functions.

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