renderer: move waveform generation into worker and mutex the waveform

Addresses scheduling issues where a backend might be closed (or even
destroyed) before it processes a waveform signal from a worker. Requires
extra multithreading code.

app/render/backend/renderbackend.cpp

@@ -34,7 +34,8 @@ OLIVE_NAMESPACE_ENTER
 RenderBackend::RenderBackend(QObject *parent) :
   QObject(parent),
   viewer_node_(nullptr),
-  update_with_graph_(false)
+  update_with_graph_(false),
+  preview_job_time_(0)
 {
 }
 
@@ -270,7 +271,6 @@ void RenderBackend::RunNextJob()
       RenderWorker* worker = CreateNewWorker();
 
       connect(worker, &RenderWorker::FinishedJob, this, &RenderBackend::WorkerFinished);
-      connect(worker, &RenderWorker::WaveformGenerated, this, &RenderBackend::WorkerGeneratedWaveform);
 
       workers_.replace(i, {worker, false});
     }
@@ -288,7 +288,9 @@ void RenderBackend::RunNextJob()
       worker->SetVideoParams(video_params_);
       worker->SetAudioParams(audio_params_);
       worker->SetVideoDownloadMatrix(video_download_matrix_);
-      worker->SetGeneratePreviews(generate_previews_);
+      if (preview_job_time_) {
+        worker->EnablePreviewGeneration(viewer_node_->audio_playback_cache(), preview_job_time_);
+      }
       worker->SetCopyMap(&copy_map_);
 
       RenderTicketPtr ticket = render_queue_.front();
@@ -351,37 +353,6 @@ void RenderBackend::WorkerFinished()
   }
 }
 
-void RenderBackend::WorkerGeneratedWaveform(const TrackOutput *copied_track, const AudioVisualWaveform& samples, const rational &r)
-{
-  if (!viewer_node_) {
-    return;
-  }
-
-  TrackOutput* track = nullptr;
-
-  /*
-  if (!viewer_node_->audio_playback_cache()->JobIsCurrent(r)) {
-    return;
-  }
-  */
-
-  QHash<Node*, Node*>::const_iterator i;
-  for (i=copy_map_.constBegin(); i!=copy_map_.constEnd(); i++) {
-    if (i.value() == copied_track) {
-      track = static_cast<TrackOutput*>(i.key());
-      break;
-    }
-  }
-
-  if (track) {
-    track->waveform().set_channel_count(audio_params_.channel_count());
-    track->waveform().OverwriteSums(samples, r);
-    emit track->PreviewChanged();
-  } else {
-    qDebug() << "Failed to find track";
-  }
-}
-
 void RenderBackend::CopyNodeInputValue(NodeInput *input)
 {
   // Find our copy of this parameter