Significantly clean up mutual recursion workaround

This has much fewer moving parts, and it's probably more understandable.
There was also a race condition in the previous implementation, so
there's that.

src/wine-host/bridges/vst3-impls/plug-frame-proxy.h

@@ -30,58 +30,10 @@ class Vst3PlugFrameProxyImpl : public Vst3PlugFrameProxy {
     tresult PLUGIN_API queryInterface(const Steinberg::TUID _iid,
                                       void** obj) override;
 
-    /**
-     * This is needed to be able to handle a call to `IPlugView::onSize()` from
-     * the UI thread while the plugin is currently calling
-     * `IPlugFrame::resizeView()` from that same thread.. This is probably the
-     * hackiest (and most error prone, probably) part of this VST3
-     * implementation. The details on how this works and why it is necessary are
-     * explained in the comment in `YaPlugFrameProxyImpl::resizeView()`.
-     *
-     * If there is currently a call to `resizeView()` being processed, then this
-     * will run `on_size` from the same thread that's currently processing a
-     * call to `resizeView()` and return the result from the function call.
-     * Otherwise this will return a nullopt and `on_size` should be passed to
-     * `main_context.run_in_context<tresult>()`.
-     */
-    template <typename F>
-    std::optional<tresult> maybe_run_on_size_from_ui_thread(F on_size) {
-        {
-            std::lock_guard lock(on_size_interrupt_mutex);
-            if (!on_size_interrupt_waiting) {
-                return std::nullopt;
-            }
-
-            on_size_interrupt = std::move(on_size);
-        }
-        on_size_interrupt_cv.notify_one();
-
-        // Since `on_size` is run from another thread, we now have to wait to be
-        // woken up again when the result is ready
-        std::unique_lock lock(on_size_interrupt_mutex);
-        on_size_interrupt_cv.wait(
-            lock, [&]() { return on_size_interrupt_result.has_value(); });
-
-        return *on_size_interrupt_result;
-    }
-
     // From `IPlugFrame`
     tresult PLUGIN_API resizeView(Steinberg::IPlugView* view,
                                   Steinberg::ViewRect* newSize) override;
 
    private:
     Vst3Bridge& bridge;
-
-    /**
-     * A function that will be used to run `IPlugView::onSize()` on the thread
-     * that originally called `IPlugFrame::resizeView()` to work around a Win32
-     * limitation, along with its result, and whether or not we're currently
-     * waiting for this function to be provided by some other thread. See the
-     * comment in `onSize()` for more information.
-     */
-    bool on_size_interrupt_waiting = false;
-    std::optional<fu2::unique_function<tresult()>> on_size_interrupt;
-    std::optional<tresult> on_size_interrupt_result;
-    std::condition_variable on_size_interrupt_cv;
-    std::mutex on_size_interrupt_mutex;
 };