Reuse YaProcessData object on the plugin side

Well, kind of. This doesn't do anything yet since the default assignment
operator will just destroy and recreate objects as normal.

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

@@ -186,12 +186,15 @@ Vst3PluginProxyImpl::process(Steinberg::Vst::ProcessData& data) {
         last_audio_thread_priority_synchronization = now;
     }
 
-    // TODO: Check whether reusing a `YaProcessData` object make a difference in
-    //       terms of performance
+    // TODO: Document
+    // TODO: Actually repopulate `process_data` with new data, right now this
+    //       assignment just destroys the old object and creates a new object.
+    process_data = data;
+
     ProcessResponse response =
         bridge.send_audio_processor_message(YaAudioProcessor::Process{
             .instance_id = instance_id(),
-            .data = data,
+            .data = process_data,
             .new_realtime_priority = new_realtime_priority});
 
     response.output_data.write_back_outputs(data);

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

@@ -490,6 +490,12 @@ class Vst3PluginProxyImpl : public Vst3PluginProxy {
      */
     std::atomic_size_t current_context_menu_id;
 
+    /**
+     * We'll reuse this process data object and simply fill the objects
+     * contained with new data to avoid allocations during audio processing.
+     */
+    YaProcessData process_data;
+
     // Caches
 
     /**