This fixes a problem where we could end up starving the redundancy
frame, especially for CBR. The solution is to make sure that some
bits are left available -- assuming we use the same rate for redundancy
as for the rest of the frame.
Only encoder changes were necessary because this uses the same
"redundant frames" mechanism as SILK<->CELT switching.
This also fixes a regression introduced in 78291b27 that was
causing the encoder to go back and forth between bandwidths when
SILK wasn't ready to change.
