bmxtranswrap: support sound only transwraps
bmxtranswrap: support sound only transwraps

- if the input is sound only and the rate is the sound sampling
rate then follow the output sample sequence if specified and
otherwise read 1920 samples per read
- Set the output clip frame rate to the timecode rate if the input
is sound only and the input rate is the sound sampling rate
- Set the timecode rate to the input frame rate if the user hasn't
set it and the input frame rate is not the sound sampling rate.
Otherwise default to 25fps

xunchangguo/BMX,ebu/ebu-bmx,stuarthicks/bmx,kierank/bmxlib-bmx,stuarthicks/bmx,kierank/bmxlib-bmx,ebu/ebu-bmx,ebu/ebu-bmx,Limecraft/ebu-bmx,kierank/bmxlib-bmx,xunchangguo/BMX,xunchangguo/BMX,Limecraft/ebu-bmx,stuarthicks/bmx,Limecraft/ebu-bmx