Encode audio as soon as we get it, instead of manually interleaving.

I honestly can't remember why this design was chosen back in the day,
but it doesn't appear to make much sense as long as the mux can interleave
for us. It (hopefully) fixes a deadlock I've seen a few times, where the
mixer hangs on begin_frame() because there are no free surfaces,
but saved_codeddata() (which is freeing them, after save) is stuck
waiting for more audio from the mixer.