MEDIUM: mux: make mux->snd_buf() take the byte count in argument

MEDIUM: mux: make mux->snd_buf() take the byte count in argument

This way the mux doesn't need to modify the buffer's metadata anymore
nor to know the output's size. The mux->snd_buf() function now takes a
const buffer and it's up to the caller to update the buffer's state.

The return type was updated to return a size_t to comply with the count
argument.