avformat/matroskaenc: Write level 1 elements in one go

Up until now, writing level 1 elements proceeded as follows: First, the
element id was written to the ordinary output AVIOContext and a dynamic
buffer was opened for the content of the level 1 element in
start_ebml_master_crc32(). Then this buffer was actually used and after it
was closed (in end_ebml_master_crc32()), the size field corresponding to
the buffer's size was written, after which the actual data was written.

This commit changes this: Nothing is written to the main AVIOContext any
more in start_ebml_master_crc32(). end_ebml_master_crc32() now writes
both the id, the length field as well as the data. This is benefical for
streaming, because a client that receives just a Cluster ID and nothing
more might infer that this is EOF; in certain usecases there is also the
danger of a client receiving the Cluster without the actual Cluster ID
at the beginning.

Addresses #8578.

(cherry picked from commit d9c21ec)

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>

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