Implement YaComponentPluginImpl destructor

When the object gets dropped through the reference counting system, the
object should also be dropped in the Wine plugin host.

src/common/serialization/vst3.h

@@ -68,7 +68,8 @@ struct WantsPluginFactory {
  * 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<YaComponent::Create, WantsPluginFactory>;
+using ControlRequest =
+    std::variant<YaComponent::Create, YaComponent::Destroy, WantsPluginFactory>;
 
 template <typename S>
 void serialize(S& s, ControlRequest& payload) {

src/common/serialization/vst3/base.h

@@ -34,3 +34,12 @@ using Steinberg::TBool, Steinberg::int8, Steinberg::int32, Steinberg::tresult;
 using ArrayUID = std::array<
     std::remove_reference_t<decltype(std::declval<Steinberg::TUID>()[0])>,
     std::extent_v<Steinberg::TUID>>;
+
+/**
+ * Empty struct for when we have send a response to some operation without any
+ * result values.
+ */
+struct Ack {
+    template <typename S>
+    void serialize(S&) {}
+};

src/common/serialization/vst3/component.h

@@ -93,6 +93,22 @@ class YaComponent : public Steinberg::Vst::IComponent {
         }
     };
 
+    /**
+     * Message to request the Wine plugin host to destroy the IComponent
+     * instance with the given instance ID. Sent from the destructor of
+     * `YaComponentPluginImpl`.
+     */
+    struct Destroy {
+        using Response = Ack;
+
+        native_size_t instance_id;
+
+        template <typename S>
+        void serialize(S& s) {
+            s.value8b(instance_id);
+        }
+    };
+
     /**
      * Instantiate this instance with arguments read from another interface
      * implementation.
@@ -136,11 +152,8 @@ class YaComponent : public Steinberg::Vst::IComponent {
     virtual tresult PLUGIN_API
     getState(Steinberg::IBStream* state) override = 0;
 
-   private:
+   protected:
     CreateArgs arguments;
-
-    // TODO: As explained in a few other places, `YaComponent` objects should be
-    //       assigned a unique ID for identification
 };
 
 template <typename S>