Move the MainContext to HostBridge

We'll use this to create a watchdog timer that shuts down the sockets
when the native host gets terminated.

src/wine-host/bridges/vst2.cpp

@@ -69,9 +69,8 @@ Vst2Bridge& get_bridge_instance(const AEffect* plugin) {
 Vst2Bridge::Vst2Bridge(MainContext& main_context,
                        std::string plugin_dll_path,
                        std::string endpoint_base_dir)
-    : HostBridge(plugin_dll_path),
+    : HostBridge(main_context, plugin_dll_path),
       logger(generic_logger),
-      main_context(main_context),
       plugin_handle(LoadLibrary(plugin_dll_path.c_str()), FreeLibrary),
       sockets(main_context.context, endpoint_base_dir, false) {
     // HACK: If the plugin library was unable to load, then there's a tiny