nodes: optimize node graph changes by only updating from inputs that changed

A huge optimization that ensures only the parts of a node graph that have
changed get pushed to the renderer. For thread-safety, the node graph is
copied elsewhere so that users can make changes asynchronously and the graph
can update when its threads are ready. Up until now, if an input value changed,
every node's values would be re-copied, or worse, if a connection was changed,
the entire graph would be recopied. This has been negligible in testing since
we've been largely testing with small graphs, but for massive projects, it's
important that this be as optimized as possible.

app/render/backend/audiorenderbackend.cpp

@@ -58,31 +58,15 @@ void AudioRenderBackend::SetParameters(const AudioRenderingParams &params)
 void AudioRenderBackend::ConnectViewer(ViewerOutput *node)
 {
   connect(node, &ViewerOutput::AudioChangedBetween, this, &AudioRenderBackend::InvalidateCache);
-  connect(node, &ViewerOutput::AudioGraphChanged, this, &AudioRenderBackend::QueueRecompile);
   connect(node, &ViewerOutput::LengthChanged, this, &AudioRenderBackend::TruncateCache);
 }
 
 void AudioRenderBackend::DisconnectViewer(ViewerOutput *node)
 {
   disconnect(node, &ViewerOutput::AudioChangedBetween, this, &AudioRenderBackend::InvalidateCache);
-  disconnect(node, &ViewerOutput::AudioGraphChanged, this, &AudioRenderBackend::QueueRecompile);
   disconnect(node, &ViewerOutput::LengthChanged, this, &AudioRenderBackend::TruncateCache);
 }
 
-bool AudioRenderBackend::CompileInternal()
-{
-  for (int i=0;i<copied_graph_.nodes().size();i++) {
-    copy_map_.insert(copied_graph_.nodes().at(i), source_node_list_.at(i));
-  }
-
-  return true;
-}
-
-void AudioRenderBackend::DecompileInternal()
-{
-  copy_map_.clear();
-}
-
 bool AudioRenderBackend::GenerateCacheIDInternal(QCryptographicHash &hash)
 {
   if (!params_.is_valid()) {
@@ -187,7 +171,7 @@ void AudioRenderBackend::ConformUnavailable(StreamPtr stream, TimeRange range, r
   } else if (audio_stream->has_conformed_version(params)) {
 
     // Index JUST finished, requeue this time
-    InvalidateCache(range);
+    InvalidateCache(range, nullptr);
 
   } else {
 
@@ -215,7 +199,7 @@ void AudioRenderBackend::ConformUpdated(Stream *stream, AudioRenderingParams par
 
       // Send invalidate cache signal
       ic_from_conform_ = true;
-      InvalidateCache(copy.affected_range);
+      InvalidateCache(copy.affected_range, nullptr);
       ic_from_conform_ = false;
 
     }