avformat/matroskaenc: Improve calculating EBML ID size

avformat/matroskaenc: Improve calculating EBML ID size

When the Matroska muxer writes an EBML ID, it calculates the length of
said ID before; and it does this as if this were a number that needs to
be encoded as EBML number: The formula used is (av_log2(id + 1) - 1) / 7
+ 1. But the constants used already contain the VINT_MARKER (the leading
bit indicating the length of the EBML number) and therefore the algorithm
used makes no sense. Instead the position of the most significant byte
set gives the desired length.

The algorithm used until now worked because EBML numbers are subject to
restrictions: If the EBML number takes up k bytes, then the bit 1 << (7
* k) is set and av_log2(id) is 7 * k. So the current algorithm produces
the correct result unless the EBML ID is of the form 7 * k - 1 because
of the "id + 1". But contrary to encoding lengths as EBML number (where
the + 1 exists to avoid the encodings reserved for unknown length),
such EBML numbers are simply forbidden as EBML IDs and as such none of
them were ever written.

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