Add dedicated IAudioProcessor/IComponent sockets

This way every relevant object instance will get its own thread for
handling these calls. The alternative would be creating a full fat
Vst3MessageHandler pair for all object instances, but that would be a
huge waste.

src/wine-host/bridges/vst3.h

@@ -38,6 +38,13 @@ struct InstanceInterfaces {
 
     InstanceInterfaces(Steinberg::IPtr<Steinberg::FUnknown> object);
 
+    /**
+     * A dedicated thread for handling incoming `IAudioProcessor` and
+     * `IComponent` calls. Will be instantiated if `object` supports either of
+     * those interfaces.
+     */
+    Win32Thread audio_processor_handler;
+
     /**
      * If the host passes a host context object during
      * `IPluginBase::initialize()`, we'll store a proxy object here and then