The bug was triggered because f982b84d started using prefill for
SILK bandwidth changes, which reinitialized the encoder state and
prevented the variable lowpass from working properly. To fix the problem,
we preserve the sampling rate and variable low-pass when prefilling.
1) In cases where the SILK desired bandwidth went down, then quickly up, we
count get stuck in a mode with the LP variation going the wrong way.
2) Bandwidth detection can no longer force SILK to go below wideband to
avoid switches that require redundancy.
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.
