[regression] refactor ebml-write to be more 0.11 friendly

[Alexey Fisher]

Hi René,
i have regression after:

commit 1b213d905ac2b9a1e015be6daf0dfeb92294154c
Author: René Stadler <rene.stadler at collabora.co.uk>
Date:   Mon Oct 10 15:36:14 2011 +0200

     matroska: refactor ebml-write to be more 0.11 friendly

     Switching to a more 0.11-friendly pattern, where getting the 
buffer's data
     pointer and setting the size many times is less natural. This is of 
course i
     preparation to the upcoming port of the plugin.



i can't play any file produced after this patch. Easyest way to 
reproduce it:
gst-launch videotestsrc num-buffers=100 ! vp8enc bitrate=100000 ! 
webmmux ! filesink location=test.webm

Regarding,
Alexey