Back the VST3 plugin factory by an IPtr

This prevents REAPER from crashing when removing the last instance of a
plugin and then readding it. REAPER doesn't unload the module even after
it removes its last plugin factory instance. This means that before this
the plugin factory would be freed but we still had a seemingly valid
pointer to it that we would try to access.

src/plugin/bridges/vst3.cpp

@@ -351,15 +351,12 @@ Vst3PluginBridge::~Vst3PluginBridge() {
 }
 
 Steinberg::IPluginFactory* Vst3PluginBridge::get_plugin_factory() {
-    // Even though we're working with raw pointers here, we should pretend that
-    // we're `IPtr<Steinberg::IPluginFactory>` and do the reference counting
-    // ourselves. This should work the same was as the standard implementation
-    // in `public.sdk/source/main/pluginfactory.h`. If we were to use an IPtr or
-    // an STL smart pointer we would get a double free (or rather, a use after
-    // free).
-    if (plugin_factory) {
-        plugin_factory->addRef();
-    } else {
+    // This works the same way as the default implementation in
+    // `public.sdk/source/main/pluginfactory.h`, with the exception that we back
+    // the plugin factory with an `IPtr` ourselves so it cannot be freed before
+    // `Vst3PluginBridge` gets freed. This is needed for REAPER as REAPER does
+    // not call `ModuleExit()`.
+    if (!plugin_factory) {
         // Set up the plugin factory, since this is the first thing the host
         // will request after loading the module. Host callback handlers should
         // have started before this since the Wine plugin host will request a
@@ -368,10 +365,14 @@ Steinberg::IPluginFactory* Vst3PluginBridge::get_plugin_factory() {
             sockets.host_vst_control.send_message(
                 YaPluginFactory::Construct{},
                 std::pair<Vst3Logger&, bool>(logger, true));
-        plugin_factory =
-            new YaPluginFactoryImpl(*this, std::move(factory_args));
+        plugin_factory = Steinberg::owned(
+            new YaPluginFactoryImpl(*this, std::move(factory_args)));
     }
 
+    // Because we're returning a raw pointer, we have to increas the
+    // reference count ourselves
+    plugin_factory->addRef();
+
     return plugin_factory;
 }