Add copy of opus library in deps/third_party.

opus/silk/bwexpander.c

+/***********************************************************************
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+***********************************************************************/
+
+#ifdef HAVE_CONFIG_H
+#include "config.h"
+#endif
+
+#include "SigProc_FIX.h"
+
+/* Chirp (bandwidth expand) LP AR filter */
+void silk_bwexpander(
+    opus_int16                  *ar,                /* I/O  AR filter to be expanded (without leading 1)                */
+    const opus_int              d,                  /* I    Length of ar                                                */
+    opus_int32                  chirp_Q16           /* I    Chirp factor (typically in the range 0 to 1)                */
+)
+{
+    opus_int   i;
+    opus_int32 chirp_minus_one_Q16 = chirp_Q16 - 65536;
+
+    /* NB: Dont use silk_SMULWB, instead of silk_RSHIFT_ROUND( silk_MUL(), 16 ), below.  */
+    /* Bias in silk_SMULWB can lead to unstable filters                                */
+    for( i = 0; i < d - 1; i++ ) {
+        ar[ i ]    = (opus_int16)silk_RSHIFT_ROUND( silk_MUL( chirp_Q16, ar[ i ]             ), 16 );
+        chirp_Q16 +=            silk_RSHIFT_ROUND( silk_MUL( chirp_Q16, chirp_minus_one_Q16 ), 16 );
+    }
+    ar[ d - 1 ] = (opus_int16)silk_RSHIFT_ROUND( silk_MUL( chirp_Q16, ar[ d - 1 ] ), 16 );
+}