avformat/matroskaenc: Avoid unnecessary seek

avformat/matroskaenc: Avoid unnecessary seek

When writing the SeekHead (a form of index) at the end of the muxing
process, mkv_write_seekhead() would first seek to the position where the
SeekHead ought to be written, then write it there and seek back to the
original position afterwards. Which means: To the end of the file.
Afterwards, a seek to the beginning of the file is performed to update
further values. This of course means that the second seek in
mkv_write_seekhead() was unnecessary.

This has been changed: A new parameter was added to mkv_write_seekhead()
containing the destination for the second seek, effectively eliminating
the seek to the end of the file after writing the SeekHead.

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