avformat/matroskaenc: Rename functions to better reflect what they do

avformat/matroskaenc: Rename functions to better reflect what they do

EBML uses variable length integers both for the EBML IDs as well as for
the EBML lengths; Matroska also uses them for the TrackNumber in
(Simple)Blocks and for the lengths of laces when EBML lacing is used.

When encoding EBML lengths, certain encodings have a special meaning,
namely that the element has an unknown length. This is not so when
encoding general EBML variable length integers.

Yet the functions called ebml_num_size() and put_ebml_num() had this
special meaning hardcoded, i.e. they are there to write EBML lengths and
not general EBML numbers. So rename them.

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