Allow indirect IConnectionPoint connections

This is needed to support Ardour. These extra hops and serialization
steps will probably hurt performance, but outside of some huge hacks (to
connect the components directly anyways) there's not much else we can
do.

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

@@ -214,17 +214,19 @@ tresult PLUGIN_API Vst3PluginProxyImpl::getState(Steinberg::IBStream* state) {
 tresult PLUGIN_API Vst3PluginProxyImpl::connect(IConnectionPoint* other) {
     // 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
+    // objects in the Wine plugin host directly. Otherwise we'll have to set up
+    // a proxy for the host's connection proxy so the messages can be routed
+    // through that.
     if (auto other_proxy = dynamic_cast<Vst3PluginProxy*>(other)) {
         return bridge.send_message(YaConnectionPoint::Connect{
-            .instance_id = instance_id(),
-            .other_instance_id = other_proxy->instance_id()});
+            .instance_id = instance_id(), .other = other_proxy->instance_id()});
     } 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;
+        connection_point_proxy = other;
+
+        return bridge.send_message(YaConnectionPoint::Connect{
+            .instance_id = instance_id(),
+            .other =
+                Vst3ConnectionPointProxy::ConstructArgs(other, instance_id())});
     }
 }
 
@@ -235,12 +237,12 @@ tresult PLUGIN_API Vst3PluginProxyImpl::disconnect(IConnectionPoint* other) {
             .instance_id = instance_id(),
             .other_instance_id = other_proxy->instance_id()});
     } else {
-        // TODO: Add support for `ConnectionProxy` and similar objects
-        bridge.logger.log(
-            "WARNING: The host passed a proxy proxy object to "
-            "'IConnectionPoint::disconnect()'. This is currently not "
-            "supported.");
-        return Steinberg::kNotImplemented;
+        const tresult result = bridge.send_message(
+            YaConnectionPoint::Disconnect{.instance_id = instance_id(),
+                                          .other_instance_id = std::nullopt});
+        connection_point_proxy.reset();
+
+        return result;
     }
 }