audiomonitor: use mipmapped waveform

Since we have these now, may as well use them. This should
improve general playback performance since the audio monitor
will be able to iterate far fewer

app/widget/viewer/viewer.cpp

@@ -412,6 +412,8 @@ void ViewerWidget::StartAudioOutput()
     AudioManager::instance()->StartOutput(audio_cache,
                                           audio_cache->GetParameters().time_to_bytes(GetTime()),
                                           playback_speed_);
+    emit AudioManager::instance()->OutputWaveformStarted(waveform_view_->waveform(),
+                                                         GetTime(), playback_speed_);
   }
 }