Further reduce allocations by reusing responses

On the plugin's side, still need to do a lot of work on the Wine side of
things.

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

@@ -220,12 +220,15 @@ Vst3PluginProxyImpl::process(Steinberg::Vst::ProcessData& data) {
     process_request.data.repopulate(data);
     process_request.new_realtime_priority = new_realtime_priority;
 
-    ProcessResponse response = bridge.send_audio_processor_message(
-        MessageReference<YaAudioProcessor::Process>(process_request));
+    // We'll also receive the response into an existing object so we can also
+    // avoid heap allocations there
+    bridge.receive_audio_processor_message_into(
+        MessageReference<YaAudioProcessor::Process>(process_request),
+        process_response);
 
-    response.output_data.write_back_outputs(data);
+    process_response.output_data.write_back_outputs(data);
 
-    return response.result;
+    return process_response.result;
 }
 
 uint32 PLUGIN_API Vst3PluginProxyImpl::getTailSamples() {

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

@@ -489,6 +489,13 @@ class Vst3PluginProxyImpl : public Vst3PluginProxy {
      */
     YaAudioProcessor::Process process_request;
 
+    /**
+     * The response object we'll get in return when we send the
+     * `process_request` object above to the Wine plugin host. This object also
+     * contains heap data, so we also want to reuse this.
+     */
+    YaAudioProcessor::ProcessResponse process_response;
+
     // Caches
 
     /**

src/plugin/bridges/vst3.h

@@ -121,6 +121,21 @@ class Vst3PluginBridge : PluginBridge<Vst3Sockets<std::jthread>> {
             object, std::pair<Vst3Logger&, bool>(logger, true));
     }
 
+    /**
+     * Send an `IAudioProcessor` or `IComponent` control message to a specific
+     * plugin instance, receiving the results into an existing object. This is
+     * similar to the `send_audio_processor_message()` above, but this lets us
+     * avoid allocations in response objects that contain heap data.
+     */
+    template <typename T>
+    typename T::Response& receive_audio_processor_message_into(
+        const T& object,
+        typename T::Response& response_object) {
+        return sockets.receive_audio_processor_message_into(
+            object, response_object,
+            std::pair<Vst3Logger&, bool>(logger, true));
+    }
+
     /**
      * The logging facility used for this instance of yabridge. Wraps around
      * `PluginBridge::generic_logger`.