Allow resizing shared memory buffers in setActive

REAPER apparently changes bus arrangements between
`IAudioProcessor::setupProcessing()` and `IAudioProcessor::setActive()`.

src/common/serialization/vst3/plugin/audio-processor.h

@@ -194,6 +194,11 @@ class YaAudioProcessor : public Steinberg::Vst::IAudioProcessor {
     /**
      * Message to pass through a call to
      * `IAudioProcessor::setupProcessing(setup)` to the Wine plugin host.
+     *
+     * Here Wine plugin host will set up the shared memory buffers.
+     *
+     * NOTE: This process is repeated as part of `SetActive`. Apparently REAPER
+     *       can change bus arrangements after the processing has been set up.
      */
     struct SetupProcessing {
         using Response = SetupProcessingResponse;

src/common/serialization/vst3/plugin/component.h

@@ -18,6 +18,7 @@
 
 #include <pluginterfaces/vst/ivstcomponent.h>
 
+#include "../../../audio-shm.h"
 #include "../../../bitsery/ext/in-place-optional.h"
 #include "../../common.h"
 #include "../base.h"
@@ -251,12 +252,32 @@ class YaComponent : public Steinberg::Vst::IComponent {
                                            int32 index,
                                            TBool state) override = 0;
 
+    /**
+     * The response code and written state for a call to
+     * `IAudioProcessor::setupProcessing(setup)`.
+     */
+    struct SetActiveResponse {
+        UniversalTResult result;
+        std::optional<AudioShmBuffer::Config> updated_audio_buffers_config;
+
+        template <typename S>
+        void serialize(S& s) {
+            s.object(result);
+            s.ext(updated_audio_buffers_config,
+                  bitsery::ext::InPlaceOptional{});
+        }
+    };
+
     /**
      * Message to pass through a call to `IComponent::setActive(state)` to the
      * Wine plugin host.
+     *
+     * NOTE: REAPER may change a plugin's bus arrangements after the processing
+     *       has been set up, so we need to check for this on every
+     *       `setActive()` call
      */
     struct SetActive {
-        using Response = UniversalTResult;
+        using Response = SetActiveResponse;
 
         native_size_t instance_id;