avformat/movenc: make AVStream easier to access

This adds a AVStream pointer to Track

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/movenc: set pretty compressor name for XDCAM

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/movenc: set XDCAM codec tag correctly

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