makewebm should write duration block for last block in cluster (or file)

In order to know the duration of the last block in a file (or perhaps in a 
cluster too), makewebm should write a BlockDuration element.

Description of BlockDuration element here:

http://www.matroska.org/technical/specs/index.html

"The duration of the Block (based on TimecodeScale). This element is mandatory 
when DefaultDuration is set for the track. When not written and with no 
DefaultDuration, the value is assumed to be the difference between the timecode 
of this Block and the timecode of the next Block in "display" order (not coding 
order). This element can be useful at the end of a Track (as there is not other 
Block available), or when there is a break in a track like for subtitle tracks."

Original issue reported on code.google.com by matthewj...@google.com on 19 Oct 2010 at 7:45

iniwf / webm

makewebm should write duration block for last block in cluster (or file) #202