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Improves CELT bit allocation at very low bitrates

We now try not to fold below band 17 since that produces a lot of harshness.
This mostly helps around 32-40 kb/s.

--- a/celt/rate.c

+++ b/celt/rate.c

@@ -348,12 +348,17 @@

             /*This if() block is the only part of the allocation function that

                is not a mandatory part of the bitstream: any bands we choose to

                skip here must be explicitly signaled.*/

-            /*Choose a threshold with some hysteresis to keep bands from

-               fluctuating in and out.*/

+            int depth_threshold;

+            /*We choose a threshold with some hysteresis to keep bands from

+               fluctuating in and out, but we try not to fold below a certain point. */

+            if (codedBands > 17)

+               depth_threshold = j<prev ? 7 : 9;

+            else

+               depth_threshold = 0;

 #ifdef FUZZING

             if ((rand()&0x1) == 0)

 #else

-            if (codedBands<=start+2 || (band_bits > ((j<prev?7:9)*band_width<<LM<<BITRES)>>4 && j<=signalBandwidth))

+            if (codedBands<=start+2 || (band_bits > (depth_threshold*band_width<<LM<<BITRES)>>4 && j<=signalBandwidth))

 #endif

             {

                ec_enc_bit_logp(ec, 1, 1);