Handle effMainsChanged on the main UI thread #75

This fixes EZdrummer not producing any sound, as the plugin presumably
schedules some task on the Win32 message loop to load its resources,
which won't happen if this is run from any other thread.

src/wine-host/bridges/vst2.cpp

@@ -40,10 +40,14 @@ std::mutex current_bridge_instance_mutex;
 /**
  * Opcodes that should always be handled on the main thread because they may
  * involve GUI operations.
+ *
+ * NOTE: `effMainsChanged` is the odd one here. EZdrummer interacts with the
+ *       Win32 message loop while handling this function. If we don't execute
+ *       this from the main GUI thread, then EZdrummer won't produce any sound.
  */
-const std::set<int> unsafe_opcodes{effOpen,     effClose,     effEditGetRect,
-                                   effEditOpen, effEditClose, effEditIdle,
-                                   effEditTop};
+const std::set<int> unsafe_opcodes{effOpen,     effClose,       effEditGetRect,
+                                   effEditOpen, effEditClose,   effEditIdle,
+                                   effEditTop,  effMainsChanged};
 
 intptr_t VST_CALL_CONV
 host_callback_proxy(AEffect*, int, int, intptr_t, void*, float);