render one frame per thread for increased parallelism

If the nodes are now stateless, there's nothing stopping the renderer from
rendering multiple frames at once. Earlier since the nodes held some of their
input/output data (and that data could change per frame), it was not possible
to render multiple frames at once without conflicts. Now that the node state is
held in render threads, they can do whatever they want at any time.

app/node/input.cpp

@@ -283,7 +283,7 @@ void NodeInput::set_maximum(const QVariant &max)
   has_maximum_ = true;
 }
 
-void NodeInput::CopyValues(NodeInput *source, NodeInput *dest, bool include_connections)
+void NodeInput::CopyValues(NodeInput *source, NodeInput *dest, bool include_connections, bool lock_connections)
 {
   Q_ASSERT(source->id() == dest->id());
 
@@ -295,7 +295,7 @@ void NodeInput::CopyValues(NodeInput *source, NodeInput *dest, bool include_conn
 
   // Copy connections
   if (include_connections && source->get_connected_output() != nullptr) {
-    ConnectEdge(source->get_connected_output(), dest);
+    ConnectEdge(source->get_connected_output(), dest, lock_connections);
   }
 
   // If these inputs are an array, copy the subparams too