Pass pointers to IMessage objects around

Instead of serializing the actual `YaMessage`, for the reasons mentioned
in the comments. This was needed to stop iZotope VocalSynth 2 in Ardour
from segfaulting when editing parameters, because that plugin is
apparently being very naughty.

src/common/serialization/vst3/message.cpp

@@ -16,6 +16,52 @@
 
 #include "message.h"
 
+YaMessagePtr::YaMessagePtr(){FUNKNOWN_CTOR}
+
+YaMessagePtr::YaMessagePtr(IMessage& message)
+    : message_id(message.getMessageID()
+                     ? std::make_optional<std::string>(message.getMessageID())
+                     : std::nullopt),
+      original_message_ptr(static_cast<native_size_t>(
+          reinterpret_cast<size_t>(&message))){FUNKNOWN_CTOR}
+
+      YaMessagePtr::~YaMessagePtr() {
+    FUNKNOWN_DTOR
+}
+
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wdelete-non-virtual-dtor"
+IMPLEMENT_FUNKNOWN_METHODS(YaMessagePtr,
+                           Steinberg::Vst::IMessage,
+                           Steinberg::Vst::IMessage::iid)
+#pragma GCC diagnostic pop
+
+Steinberg::Vst::IMessage* YaMessagePtr::get_original() const {
+    // See the docstrings on `YaMessage` and `YaMessagePtr`
+    return reinterpret_cast<IMessage*>(
+        static_cast<size_t>(original_message_ptr));
+}
+
+Steinberg::FIDString PLUGIN_API YaMessagePtr::getMessageID() {
+    if (message_id) {
+        return message_id->c_str();
+    } else {
+        return nullptr;
+    }
+}
+
+void PLUGIN_API YaMessagePtr::setMessageID(Steinberg::FIDString id /*in*/) {
+    if (id) {
+        message_id = id;
+    } else {
+        message_id.reset();
+    }
+}
+
+Steinberg::Vst::IAttributeList* PLUGIN_API YaMessagePtr::getAttributes() {
+    return &attribute_list;
+}
+
 YaMessage::YaMessage(){FUNKNOWN_CTOR}
 
 YaMessage::~YaMessage() {