Allow mutual recursion between audioMasterUpdateDisplay and effGetProgram

This prevents Voxengo VST2 plugins from freezing in Renoise when
recalling their state. See the comments in the diff for why this is
necessary.

src/wine-host/bridges/vst2.h

@@ -23,6 +23,7 @@
 
 #include "../../common/communication/vst2.h"
 #include "../../common/configuration.h"
+#include "../../common/mutual-recursion.h"
 #include "../editor.h"
 #include "common.h"
 
@@ -205,4 +206,18 @@ class Vst2Bridge : public HostBridge {
      * now happens in two different threads.
      */
     std::mutex next_buffer_midi_events_mutex;
+
+    /**
+     * Used to allow the responses to host callbacks to be handled on the same
+     * thread that originally made the callback. Luckily this is much rarer with
+     * VST2 plugins than with VST3 plugins (presumably because plugins make
+     * fewer GUI-related-ish callbacks), but it does happen.
+     *
+     * See `mutually_recursive_callbacks` and
+     * `safe_mutually_recursive_requests` in the implementation file for more
+     * information on the callbacks where we want to use
+     * `mutual_recursion.fork()` to send them in a new thread, and the responses
+     * that have to be handled with `mutual_recursion.handle()`.
+     */
+    MutualRecursionHelper<Win32Thread> mutual_recursion;
 };