Only initialize VST2 audio buffers on activation

Ardour apparently always calls `effMainsChanged()` with a value argument
of 0 when unloading the plugin, regardless of whether it has actually
initialized audio processing before that point.

src/plugin/bridges/vst2.cpp

@@ -227,8 +227,16 @@ class DispatchDataConverter : public DefaultDataConverter {
                 break;
             case effMainsChanged:
                 // At this point we'll set up our audio buffers since we (in
-                // theory) now know how large they need to be
-                return WantsAudioShmBufferConfig{};
+                // theory) now know how large they need to be. A value argument
+                // of 1 means that audio playback should be initialized.
+                // NOTE: Ardour unconditionally calls this with a value of 0
+                //       unconditionally when unloading a plugin, even if it has
+                //       never initialized audio playback
+                if (value == 1) {
+                    return WantsAudioShmBufferConfig{};
+                } else {
+                    return nullptr;
+                }
             case effEditGetRect:
                 return WantsVstRect();
                 break;
@@ -367,12 +375,14 @@ class DispatchDataConverter : public DefaultDataConverter {
                 update_aeffect(plugin, updated_plugin);
             } break;
             case effMainsChanged: {
-                const auto& audio_buffer_config =
-                    std::get<AudioShmBuffer::Config>(response.payload);
-                if (!process_buffers) {
-                    process_buffers.emplace(audio_buffer_config);
-                } else {
-                    process_buffers->resize(audio_buffer_config);
+                if (const auto* audio_buffer_config =
+                        std::get_if<AudioShmBuffer::Config>(
+                            &response.payload)) {
+                    if (!process_buffers) {
+                        process_buffers.emplace(*audio_buffer_config);
+                    } else {
+                        process_buffers->resize(*audio_buffer_config);
+                    }
                 }
             } break;
             case effEditGetRect: {

src/wine-host/bridges/vst2.cpp

@@ -470,7 +470,11 @@ void Vst2Bridge::run() {
             // the same buffers. We cannot use `Vst2Bridge::dispatch_wrapper()`
             // for this because we need to directly return payload data that
             // won't be visible to the plugin at all.
-            if (event.opcode == effMainsChanged) {
+            // NOTE: Ardour will call `effMainsChanged()` with a value of 1
+            //       unconditionally when unloading a plugin, even when audio
+            //       playback has never been initialized (and `effSetBlockSize`
+            //       has never been called)
+            if (event.opcode == effMainsChanged && event.value == 1) {
                 // Returning another result this way is a bit ugly, but sadly
                 // optimizations have never made code nicer to read
                 return Vst2EventResult{.return_value = result.return_value,