matroskaenc: Write muxingapp and writingapp elements when using bitexact flag

Files won't validate with mkvalidtor if these two elements are missing.
Use a const "Lavf" string that wont change with library version bumps.

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
