Up until now, the SimpleBlock was treated specially: It basically had
its own EBML category and it was also included in the BlockGroup EBML
syntax (although a SimpleBlock must not exist in a BlockGroup according
to the Matroska specifications). The latter fact also meant that
a MatroskaBlock's buffer was always unreferenced twice.
This has been changed: The type of a SimpleBlock is now an EBML_BIN.
The only way in which SimpleBlocks are still different is that they
share their associated structure with another unit (namely BlockGroup).
This is also used to unref the block: It is always unreferenced via the
BlockGroup syntax.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>