libavfilter: improve atempo audio quality

Reduce audio fragment alignment jitter by penalizing alignment
correction offsets that deviate too much from the target offset.
This is accomplished by multiplying the cross correlation search
window with a quadratic function.
Signed-off-by: Pavel Koshevoy <pavel@homestead.aragog.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavfilter/af_atempo.c

@@ -654,7 +654,7 @@ static int yae_align(AudioFragment *frag,
 
         // normalize:
         FFTSample drifti = (FFTSample)(drift + i);
-        metric *= drifti;
+        metric *= drifti * (FFTSample)(i - i0) * (FFTSample)(i1 - i);
 
         if (metric > best_metric) {
             best_metric = metric;