Apply logarithmic scaling

To match perceived loudness

src/minidexed.cpp

@@ -1793,6 +1793,9 @@ void CMiniDexed::setMasterVolume (float32_t vol)
     else if (vol > 1.0)
         vol = 1.0;
 
+    // Apply logarithmic scaling to match perceived loudness
+    vol = powf(vol, 2.0f);
+
     nMasterVolume = vol;
 }