Bypass connection point proxies when possible

This greatly improves compatibility with VST3 plugins in Ardour and
Mixbus. Most notably the FabFilter plugins would previously freeze when
having the GUI open while duplicating or inserting new instances.

src/plugin/bridges/vst3-impls/plugin-proxy.h

@@ -324,10 +324,21 @@ class Vst3PluginProxyImpl : public Vst3PluginProxy {
     Steinberg::IPtr<Steinberg::Vst::IComponentHandler> component_handler;
 
     /**
-     * If the host doesn't connect two objects directly in
-     * `IConnectionPoint::connect` but instead connects them through a proxy,
-     * we'll store that proxy here. This way we can then route messages sent by
-     * the plugin through this proxy. So far this is only needed for Ardour.
+     * If the host places a proxy between two objects in
+     * `IConnectionPoint::connect()`, we'll first try to bypass this proxy to
+     * avoid a lot of edge cases with plugins that use these notifications from
+     * the GUI thread. We'll do this by exchanging messages containing the
+     * connected object's instance ID. If we can successfully exchange instance
+     * IDs this way, we'll still connect the objects directly on the Wine plugin
+     * host side. So far this is only needed for Ardour.
+     */
+    std::optional<size_t> connected_instance_id;
+
+    /**
+     * If we cannot manage to bypass the connection proxy as mentioned in the
+     * docstring of `connected_instance_id`, then we'll store the host's
+     * connection point proxy here and we'll proxy that proxy, if that makes any
+     * sense.
      */
     Steinberg::IPtr<Steinberg::Vst::IConnectionPoint> connection_point_proxy;