avformat/movenc: write reel_name metadata to tmcd atom

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/matroskaenc: fix MAX_CUEPOINT_SIZE calculation

Fixes assertion failure
Fixes Ticket3822

as a side-effect this makes some mkv files a few bytes smaller

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>