Store whether two objects are connected directly

In the host context. So when the plugin wants to create an `IMessage`
object to send a message to the other object, we don't have to go
through the host.

src/common/serialization/vst3/host-context-proxy.h

@@ -97,6 +97,15 @@ class Vst3HostContextProxy : public YaHostApplication {
         return arguments.owner_instance_id;
     }
 
+    /**
+     * Used to shortcut calls to
+     * `IHostApplication::createInstance(IMessage::iid, IMessage::iid, &obj)`
+     * when two objects (a processor and a controller instance, for example) are
+     * directly connected. This way we don't have to proxy the message created
+     * by the host, which can save a lot of resoruces.
+     */
+    std::atomic_bool are_objects_directly_connected = false;
+
    private:
     ConstructArgs arguments;
 };