avformat/matroskaenc: Reduce usage of ebml_master

After the last few commits, the functions for writing master elements
with CRC-32 elements didn't really make use of the ebml_master
structure any more, so remove these parameters from the functions.

The only things that still need to be kept are the positions of the
level 1 elements that are written preliminarily and updated later.
These positions are stored in the MatroskaMuxContext and
replace the corresponding ebml_master structures.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>