GUI: add a preliminary oscilloscope

src/engine/playback.cpp

@@ -1138,5 +1138,10 @@ void DivEngine::nextBuf(float** in, float** out, int inChans, int outChans, unsi
     metroPos+=metroFreq;
     while (metroPos>=1) metroPos--;
   }
+
+  // TODO: improve
+  memcpy(oscBuf[0],out[0],size*sizeof(float));
+  memcpy(oscBuf[1],out[1],size*sizeof(float));
+  oscSize=size;
   isBusy.unlock();
 }