Refactor plugin factories into Vst3*Proxy format

Now every proxy object that's directly created by the host or plugin
shares the same structure.

src/common/serialization/vst3/plugin-factory-proxy.h

@@ -0,0 +1,86 @@
+// yabridge: a Wine VST bridge
+// Copyright (C) 2020-2021 Robbert van der Helm
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+#pragma once
+
+#include "../common.h"
+#include "plugin-factory/plugin-factory.h"
+
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wnon-virtual-dtor"
+
+/**
+ * An abstract class that `IPluginFactory`, and optionally also
+ * `IPluginFactory2` and `IPluginFactory3` depending on what the Windows VST3
+ * plugin's plugin factory supports. All information is read once the Wine
+ * plugin host side, so the only callbacks that we'll make from here are to
+ * create new objects and to set a host context for the factory (if the host and
+ * the plugin supports that).
+ */
+class Vst3PluginFactoryProxy : public YaPluginFactory3 {
+   public:
+    /**
+     * These are the arguments for constructing a `Vst3PluginFactoryProxyImpl`.
+     */
+    struct ConstructArgs {
+        ConstructArgs();
+
+        /**
+         * Read from an existing object. We will try to mimic this object, so
+         * we'll support any interfaces this object also supports.
+         */
+        ConstructArgs(Steinberg::IPtr<FUnknown> object);
+
+        YaPluginFactory3::ConstructArgs plugin_factory_args;
+
+        template <typename S>
+        void serialize(S& s) {
+            s.object(plugin_factory_args);
+        }
+    };
+
+    /**
+     * Message to request the Windows VST3 plugin's plugin factory information
+     * from the Wine plugin host.
+     */
+    struct Construct {
+        using Response = ConstructArgs;
+
+        template <typename S>
+        void serialize(S&) {}
+    };
+
+    /**
+     * Instantiate this instance with arguments read from an actual plugin
+     * factory. The is done once during startup and the plugin factory gets
+     * reused for the lifetime of the module.
+     */
+    Vst3PluginFactoryProxy(const ConstructArgs&& args);
+
+    /**
+     * We do not need special handling here since the Window VST3 plugin's
+     * plugin factory will also be destroyed when we terminate the Wine plugin
+     * host or unload the plugin there.
+     */
+    virtual ~Vst3PluginFactoryProxy();
+
+    DECLARE_FUNKNOWN_METHODS
+
+   private:
+    ConstructArgs arguments;
+};
+
+#pragma GCC diagnostic pop