Pass pointers to IMessage objects around

Instead of serializing the actual `YaMessage`, for the reasons mentioned
in the comments. This was needed to stop iZotope VocalSynth 2 in Ardour
from segfaulting when editing parameters, because that plugin is
apparently being very naughty.

src/wine-host/bridges/vst3-impls/connection-point-proxy.cpp

@@ -55,15 +55,14 @@ Vst3ConnectionPointProxyImpl::disconnect(IConnectionPoint* /*other*/) {
 
 tresult PLUGIN_API
 Vst3ConnectionPointProxyImpl::notify(Steinberg::Vst::IMessage* message) {
-    // As explained in `YaMessage` and `Vst3PluginProxyImpl::notify`, we can
-    // only support our own `IMessage implementation here`
-    if (auto message_impl = dynamic_cast<YaMessage*>(message)) {
-        return bridge.send_message(YaConnectionPoint::Notify{
-            .instance_id = owner_instance_id(), .message = *message_impl});
+    if (message) {
+        return bridge.send_message(
+            YaConnectionPoint::Notify{.instance_id = owner_instance_id(),
+                                      .message_ptr = YaMessagePtr(*message)});
     } else {
-        std::cerr << "WARNING: Unknown message type passed to "
+        std::cerr << "WARNING: Null pointer passed to "
                      "'IConnectionPoint::notify()', ignoring"
                   << std::endl;
-        return Steinberg::kNotImplemented;
+        return Steinberg::kInvalidArgument;
     }
 }

src/wine-host/bridges/vst3.cpp

@@ -223,21 +223,21 @@ void Vst3Bridge::run() {
                     return result;
                 }
             },
-            [&](YaConnectionPoint::Notify& request)
+            [&](const YaConnectionPoint::Notify& request)
                 -> YaConnectionPoint::Notify::Response {
-                // FIXME: This needs to be redesigned. We should only send a
-                //        pointer to the `IMessage` instead of copying the
-                //        actual message. What ends up happening is that iZotope
-                //        VocalSynth 2 exchanges pointers to the processor and
-                //        the controller. Then at some point during an
-                //        `IAudioProcessor::process()` call after a parameter
-                //        changes, it will try to access the pointers stored in
-                //        that message. So to be able to support that, the
-                //        message object we pass to notify here still has to be
-                //        alive at that point. This goes 100% against the design
-                //        of VST3, but there's nothing we can do about it.
+                // NOTE: We're using a few tricks here to pass through a pointer
+                //       to the _original_ `IMessage` object passed to a
+                //       connection proxy. This is needed because some plugins
+                //       like iZotope VocalSynth 2 use these messages to
+                //       exchange pointers between their objects so they can
+                //       break out of VST3's separation, but they might also
+                //       store the message object and not the actual pointers.
+                //       We should thus be passing a (raw) pointer to the
+                //       original object so we can pretend none of this wrapping
+                //       and serializing has ever happened.
                 return object_instances[request.instance_id]
-                    .connection_point->notify(&request.message);
+                    .connection_point->notify(
+                        request.message_ptr.get_original());
             },
             [&](YaEditController::SetComponentState& request)
                 -> YaEditController::SetComponentState::Response {