lavfi/avf_showspectrum: replace pow(x, 0.25) by sqrt(sqrt(x))

This is faster; precision assured as result is a float.
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>

libavfilter/avf_showspectrum.c

@@ -590,7 +590,7 @@ static int plot_spectrum_column(AVFilterLink *inlink, AVFrame *insamples)
                 a = cbrt(a);
                 break;
             case FOURTHRT:
-                a = pow(a, 0.25);
+                a = sqrt(sqrt(a));
                 break;
             case FIFTHRT:
                 a = pow(a, 0.20);