Using a finer table for the rate to SNR curves in silk_control_SNR().
It's now possible to have an SNR that reaches 0, so we can lower bitrate
down to ~5 kbps for narrowband and 5.5 kbps for wideband.
The bug was caused by an improper feedback of the per-frame bitrate, causing
the bitrate to jump up and down from frame to frame, within a packet.
The patch avoids this, and also gives a slight improvement in general for
multi-frame packets, even without FEC.
Signed-off-by: Jean-Marc Valin <jmvalin@jmvalin.ca>
