matroska: Reverse playback not working with webm file

Submitted by Xavi Artigas @xartigas

Link to original bug (#679250)

Description

Created attachment 217815
Basic tutorial 13 from the GStreamer SDK tutorials

The attached code exercises multiple trick modes.
With a webm file, frame stepping and fast forward seem to work correctly, but, going backwards (press 'D') is completely ignored for HTTP files and hangs with this message for local files:
CRITICAL **: gst_matroska_demux_seek_to_previous_keyframe: assertion `demux->seek_index' failed

gst-playback-test is reported as not working nicely either.

Attachment 217815, "Basic tutorial 13 from the GStreamer SDK tutorials":
basic-tutorial-13.c