Use mutual recursion for IEditController::setState

With this we should be able to handle `setState()`s that try to resize
the currently open editor. This could pop up when using the preset
browser in REAPER with plugins that recall their old size when loading a
preset.

src/plugin/bridges/vst3-impls/plugin-proxy.h

@@ -354,6 +354,12 @@ class Vst3PluginProxyImpl : public Vst3PluginProxy {
      */
     Vst3PlugViewProxyImpl* last_created_plug_view = nullptr;
 
+    /**
+     * Whether `last_created_plug_view` is currently active. This field is
+     * written to from `Vst3PlugViewProxyImpl`'s constructor and destructor.
+     */
+    std::atomic_bool last_created_plug_view_active = false;
+
     /**
      * A pointer to a context menu returned by the host as a response to a call
      * to `IComponentHandler3::createContextMenu`, as well as all targets we've