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Adds a ramp up in the SILK prefill

This improves quality of CELT->SILK switches by not having the SILK encoder
attempt to encode a discontinuity. Overall, it's a small PQ improvement,
confirmed by some listening.

--- a/src/opus_encoder.c

+++ b/src/opus_encoder.c

@@ -1087,6 +1087,20 @@

         if (prefill)

         {

             opus_int32 zero=0;

+            const CELTMode *celt_mode;

+            int prefill_offset;

+            celt_encoder_ctl(celt_enc, CELT_GET_MODE(&celt_mode));

+            /* Use a smooth onset for the SILK prefill to avoid the encoder trying to encode

+               a discontinuity. The exact location is what we need to avoid leaving any "gap"

+               in the audio when mixing with the redundant CELT frame. Here we can afford to

+               overwrite st->delay_buffer because the only thing that uses it before it gets

+               rewritten is tmp_prefill[] and even then only the part after the ramp really

+               gets used (rather than sent to the encoder and discarded) */

+            prefill_offset = st->channels*(st->encoder_buffer-st->delay_compensation-st->Fs/400);

+            gain_fade(st->delay_buffer+prefill_offset, st->delay_buffer+prefill_offset,

+                  0, Q15ONE, celt_mode->overlap, st->Fs/400, st->channels, celt_mode->window, st->Fs);

+            for(i=0;i<prefill_offset;i++)

+               st->delay_buffer[i]=0;

 #ifdef FIXED_POINT

             pcm_silk = st->delay_buffer;

 #else