matroskademux: add support for reverse playback in files without index

Submitted by Farkas Levente `@lfarkas`

Created attachment 273989
proposed patch

here is our proposed patch to matroska-demux.c to be able to play backwards in a not indexed (not closed) file.

this is for 0.10.36 but probably the same patch can be applied to the trunk too.

any comments are welcome.

Patch 273989, "proposed patch ":
matroska-demux-seek_backwards.patch