Optimize factor -> db conversions, this improves decoding speed.

Signed-off-by: Stefan Westerfeld <stefan@space.twc.de>

src/audiowmark.cc

@@ -224,12 +224,23 @@ window_hamming (double x) /* sharp (rectangle) cutoffs at boundaries */
   return 0.54 + 0.46 * cos (M_PI * x);
 }
 
+/*
+ * glibc log2f is a lot faster than glibc log10
+ */
+inline double
+log10_approx (double l)
+{
+  constexpr double log2_log10_factor = 0.3010299956639811952; // 1 / log2 (10)
+
+  return log2f (l) * log2_log10_factor;
+}
+
 double
 db_from_factor (double factor, double min_dB)
 {
   if (factor > 0)
     {
-      double dB = log10 (factor); /* Bell */
+      double dB = log10_approx (factor); /* Bell */
       dB *= 20;
       return dB;
     }