Drop IPlugView pointer when host drops proxy

src/common/serialization/vst3.h

@@ -59,7 +59,8 @@ struct WantsConfiguration {
  * encodes the information we request or the operation we want to perform. A
  * request of type `ControlRequest(T)` should send back a `T::Response`.
  */
-using ControlRequest = std::variant<Vst3PluginProxy::Construct,
+using ControlRequest = std::variant<Vst3PlugViewProxy::Destruct,
+                                    Vst3PluginProxy::Construct,
                                     Vst3PluginProxy::Destruct,
                                     Vst3PluginProxy::SetState,
                                     Vst3PluginProxy::GetState,

src/common/serialization/vst3/plug-view-proxy.h

@@ -72,6 +72,22 @@ class Vst3PlugViewProxy : public YaPlugView {
      */
     Vst3PlugViewProxy(const ConstructArgs&& args);
 
+    /**
+     * Message to request the Wine plugin host to destroy the `IPlugView*`
+     * returned by the object with the given instance ID. Sent from the
+     * destructor of `Vst3PlugViewProxyImpl`.
+     */
+    struct Destruct {
+        using Response = Ack;
+
+        native_size_t owner_instance_id;
+
+        template <typename S>
+        void serialize(S& s) {
+            s.value8b(owner_instance_id);
+        }
+    };
+
     /**
      * @remark The plugin side implementation should send a control message to
      *   clean up the instance on the Wine side in its destructor.