Apply new compensation only if it changed

If the compensation is the same (typically when it is 0), do not reapply
it.

app/src/audio_player.h

@@ -60,6 +60,9 @@ struct sc_audio_player {
     // (protected by SDL_AudioDeviceLock())
     uint32_t underflow;
 
+    // Current applied compensation value (only used by the receiver thread)
+    int compensation;
+
     // Set to true the first time a sample is received (protected by
     // SDL_AudioDeviceLock())
     bool received;