Add LP zero synthesis filter. Patch by Kenan Gillet.

Originally committed as revision 18616 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/celp_filters.c

@@ -103,3 +103,23 @@ void ff_celp_lp_synthesis_filterf(
             out[n] -= filter_coeffs[i-1] * out[n-i];
     }
 }
+
+void ff_celp_lp_zero_synthesis_filterf(
+        float *out,
+        const float* filter_coeffs,
+        const float* in,
+        int buffer_length,
+        int filter_length)
+{
+    int i,n;
+
+    // Avoids a +1 in the inner loop.
+    filter_length++;
+
+    for(n=0; n<buffer_length; n++)
+    {
+        out[n] = in[n];
+        for(i=1; i<filter_length; i++)
+            out[n] -= filter_coeffs[i-1] * in[n-i];
+    }
+}