Handle connection point proxy from GUI thread

FabFilter plugins will exchange messages that have to be handled from
the GUI thread, or they'll get stuck waiting on a synchronisation
object. This probably hurts GUI performance significantly but luckily it
only affects Ardour.

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

@@ -52,7 +52,11 @@ Vst3ConnectionPointProxyImpl::disconnect(IConnectionPoint* /*other*/) {
 tresult PLUGIN_API
 Vst3ConnectionPointProxyImpl::notify(Steinberg::Vst::IMessage* message) {
     if (message) {
-        return bridge.send_message(
+        // FabFilter plugins require this to be done from the GUI thread so we
+        // need to use our mutual recursion mechanism. Luckily only Ardour uses
+        // connection proxies, so if this ends up breaking something it will
+        // only affect Ardour.
+        return bridge.send_mutually_recursive_message(
             YaConnectionPoint::Notify{.instance_id = owner_instance_id(),
                                       .message_ptr = YaMessagePtr(*message)});
     } else {

src/wine-host/bridges/vst3.cpp

@@ -280,9 +280,17 @@ void Vst3Bridge::run() {
                 //       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_ptr.get_original());
+                // NOTE: FabFilter plugins require some of their messages to be
+                //       handled from the GUI thread. This could make the GUI
+                //       much slower in Ardour, but there's no other non-hacky
+                //       solution for this (and bypassing Ardour's connection
+                //       proxies sort of goes against the idea behind yabridge)
+                return do_mutual_recursion_or_handle_in_main_context<tresult>(
+                    [&]() {
+                        return object_instances[request.instance_id]
+                            .connection_point->notify(
+                                request.message_ptr.get_original());
+                    });
             },
             [&](YaContextMenuTarget::ExecuteMenuItem& request)
                 -> YaContextMenuTarget::ExecuteMenuItem::Response {