💥 Reimplement VST2 audio processing

We now use shared memory to store the input and output audio buffers.
This means that we have to copy less data every processing cycle, since
a single copy to and a single copy from the shared memory object
suffices now. This should reduce the DSP load for VST2
plugins (especially when used in a plugin group) marginally to
significantly depending on the plugins used and the system
configuration.

src/common/communication/vst2.h

@@ -487,6 +487,13 @@ Vst2EventResult passthrough_event(AEffect* plugin,
             // after the plugin has already finished initializing.
             return nullptr;
         },
+        [](const WantsAudioShmBufferConfig&) -> void* {
+            // This is another magic value. We'll create the shared memory
+            // object after the plugin's dispatch function has returned and then
+            // return the configuration to the native plugin, in
+            // `Vst2Bridge::run`.
+            return nullptr;
+        },
         [&](const WantsChunkBuffer&) -> void* { return string_buffer.data(); },
         [](VstIOProperties& props) -> void* { return &props; },
         [](VstMidiKeyName& key_name) -> void* { return &key_name; },