Add boilerplate for connecting Vst3PluginProxies

This way we can identify the actual objects and directly connect them on
the Wine side.

src/common/serialization/vst3/plugin-proxy.h

@@ -154,6 +154,12 @@ class Vst3PluginProxy : public YaAudioProcessor,
 
     DECLARE_FUNKNOWN_METHODS
 
+    /**
+     * Get this object's instance ID. Used in `IConnectionPoint` to identify and
+     * connect specific objects.
+     */
+    inline size_t instance_id() const { return arguments.instance_id; }
+
     // We'll define messages for functions that have identical definitions in
     // multiple interfaces below. When the Wine plugin host process handles
     // these it should check which of the interfaces is supported on the host.

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

@@ -189,9 +189,24 @@ tresult PLUGIN_API Vst3PluginProxyImpl::getState(Steinberg::IBStream* state) {
 }
 
 tresult PLUGIN_API Vst3PluginProxyImpl::connect(IConnectionPoint* other) {
-    // TODO: Implement
-    bridge.logger.log("TODO IConnectionPoint::connect()");
-    return Steinberg::kNotImplemented;
+    // When the host is trying to connect two plugin proxy objects, we can just
+    // identify the other object by its instance IDs and then connect the
+    // objects in the Wine plugin host directly
+    if (auto other_proxy = dynamic_cast<Vst3PluginProxy*>(other)) {
+        // TODO: Remove debug
+        bridge.logger.log("Host is trying to connect us with instance " +
+                          std::to_string(other_proxy->instance_id()));
+
+        // TODO: Implement
+        bridge.logger.log("TODO IConnectionPoint::connect()");
+        return Steinberg::kNotImplemented;
+    } else {
+        // TODO: Add support for `ConnectionProxy` and similar objects
+        bridge.logger.log(
+            "WARNING: The host passed a proxy proxy object to "
+            "'IConnectionPoint::connect()'. This is currently not supported.");
+        return Steinberg::kNotImplemented;
+    }
 }
 
 tresult PLUGIN_API Vst3PluginProxyImpl::disconnect(IConnectionPoint* other) {