Do not attach visualization on startup. (#5841)

@hubertp has reported in #5620 that sometimes enabling visualization does not send "attachVisualization" message to the engine.

The actual cause was simply because it was already attached. Updating the default visualizations (when receiving information about type) updated the preprocessors, what caused in turn attaching visualization.

That was a bug, of course. This PR fixes it: now we don't update any visualization if it's hidden.

Cargo.toml

@@ -128,6 +128,7 @@ syn = { version = "1.0", features = [
   "extra-traits",
   "printing",
   "parsing",
+  "visit",
   "visit-mut",
 ] }
 quote = { version = "1.0.23" }

app/gui/view/graph-editor/src/lib.rs

@@ -1619,12 +1619,16 @@ impl GraphEditorModelWithNetwork {
 
             selected    <- vis_is_selected.on_true();
             deselected  <- vis_is_selected.on_false();
-            output.visualization_preprocessor_changed <+
-                node_model.visualization.frp.preprocessor.map(move |preprocessor|
-                    (node_id,preprocessor.clone()));
             output.on_visualization_select <+ selected.constant(Switch::On(node_id));
             output.on_visualization_select <+ deselected.constant(Switch::Off(node_id));
 
+            preprocessor_changed <-
+                node_model.visualization.frp.preprocessor.map(move |preprocessor| {
+                    (node_id,preprocessor.clone())
+                });
+            output.visualization_preprocessor_changed <+ preprocessor_changed.gate(&node.visualization_visible);
+
+
             metadata <- any(...);
             metadata <+ node_model.visualization.frp.preprocessor.map(visualization::Metadata::new);