Always handle IPlugView::onSize() from UI thread

This requires a super hacky workaround because the UI thread can be
currently blocked by the plugin calling `IPlugFrame::resizeView()` from
the Win32 message loop.

src/wine-host/bridges/vst3.h

@@ -25,6 +25,9 @@
 #include "../editor.h"
 #include "common.h"
 
+// Forward declarations
+class Vst3PlugFrameProxyImpl;
+
 /**
  * A holder for plugin object instance created from the factory. This stores all
  * relevant interface smart pointers to that object so we can handle control
@@ -72,6 +75,13 @@ struct InstanceInterfaces {
      */
     Steinberg::IPtr<Vst3PlugFrameProxy> plug_frame_proxy;
 
+    /**
+     * An unmanaged raw pointer for the actual implementation behind
+     * `plug_frame_proxy`. This is needed for some special handling for
+     * `IPlugView::onSize()`.
+     */
+    Vst3PlugFrameProxyImpl* plug_frame_proxy_impl;
+
     /**
      * The base object we cast from.
      */